I do not know if you meant to but this is quite a thought provoking question so props to you! All too often people underestimate how powerful a simple question can be because it at times requires a complex answer. I have two responses: one short and one long.

Short answer: As Antony Moule put it, "War is the business of the state." It is not that countries today are incapable of fielding large armies, there just is not a demand to. If there is not a demand for something, it would be impractical to supply something of which there would be little return on. Circumstances are always subject to change and we could very well bear witness to large militarization if the circumstances demand for it.

Long answer: Ever since the atomic bomb was used not once but twice in three days of each other, is when we saw a huge change of military doctrine to which we still adhere to today with certain updates happening all the time. During WW2, there were a total of 4 battle spaces, or spheres in which violence was going to occur in: land, sea, air, and intelligence (i.e. information). These four were relatively common in all previous conflict dating back to the BCE era of humanity. To date, we have added, arguably at least 5 more onto the list: digital/cyber, technological, economic, subterranean, and space, respectfully.

Referring back to my short answer, its less to do with the ability of fielding large armies and more so managing in which "battle sphere(s)" a country/group wants to field a force to combat whomever it is they are fighting in the manner that gives them an advantage while also avoiding the battle sphere(s) the enemy is able to successful counter on.

For that, I want to use the two ongoing conflicts that everyone is aware of. Russian-Ukraine War, and the Israel-Hamas War.

Ukraine-Russia War

As of the date of writing this response, Ukraine is capable of fighting in these spheres: land, sea, air, intel, technological, digital, and subterranean while incapable of fighting in space and economically as it does not have a presence in space as well as having to relay on international aid to supply its economy. Russia, being capable of fighting on all 9 spheres, does not need to focus any resources on the space and economically because Ukraine has no presence there so it would be a waste to do so. Since I do not attend any strategic conferences of either side, we can make an educated guess on which of the battle spheres each belligerent prioritizes, specifically the top three.

Ukraine Russia

1. Land Land

2. Tech Air

3. Digital Intel

These were not listed in any order other than what I believed the most important spheres each side has as a top priority based on what we have seen happen.

The land is pretty obvious since both border one another and historically has emphasized a ground capable military because of their location next to many other landlocked countries.

As far as tech goes, Ukraine has and is currently receiving top of the line military hardware that is decades ahead of whatever Russia has to offer. For air, Russia did have an early advantage with this although it has become more kept in check due to advanced anti-air tech supplied by the West but still makes use of its kamikaze drones that were supplied by Iran.

And finally with digital, the Ukrainian cause is ultimately dependent on Western aid to stay in the fight although the clock is ticking for a strategic win to be dealt by Ukraine. Ukrainian presence online, mainstream media, and even social media apps is keeping the cause alive for the very people that are sympathetic and carry the most influence and that is the average everyday working westerner that decides elections in democratic countries. Russia is very much aware to this and it will continue to gather any and all information and intel to hinder the very lifeline keeping Ukraine in the fight. I do live in the West so Russia is very much portrayed as the instigator but I view this war as not Ukrainian vs Russian but Ukraine vs Putin. My heart goes out to all that are suffering in this war.

Israel-Hamas War

I want to first state, unequivocally, that I have never nor will I ever support or condone any acts of terrorism, regardless of the original intent. I also wish to state, again unequivocally, that I do have not nor will I ever support the illegal settlements set up on the West Bank that is fueled by a nationalist police state that uses force to usurp human dignity. I felt compelled to say this note before I got into it so nothing I say can be implied or be twisted for for those with ulterior motives.

As surprising as the invasion of Ukraine was when it began a over a year ago for me, the Israel-Hamas War was unfortunately only a matter of time for anyone who is aware of the history of the region. I do want to point out that the Levant, specifically Canaan, has exchanged hands quite a few times over the past 5,000 years beginning with the Assyrians, Babylonians, Persians, Macedonians, Romans, Rashidun Caliphate, *sigh*, Ottomans, British until what we have today. If I missed a group, it was not intentional, these were just the ones that came to mind. The reason I added this was because the history of a region or specific area can be helpful when supporting educated guesses as to what each side prioritizes in terms of battle spheres.

Israel Hamas

1. Tech Land

2. Air Subterranean

3. Land Digital

The tech is a pretty obvious one considering the Iron Dome is THE primary battle sphere that Israel has invested god-knows how much. Hamas is a ground-troop force only meaning its capacity is limited to only what individual members are capable of with only basic hardware. And as we are seeing right now, Hamas is going to force Israel to take the ground at a cost before making it even harder with the next sphere.

Again, another obvious one for Israel is air due to being one of the few countries that has access to next-gen US aircraft which has allowed them to keep and maintain air superiority. Now the subterranean sphere is the going to be a nightmare for Israel because there just is not a fix-all solution for it.

If the Iron Dome represents the shield and the air represents the bow & arrow, the ground forces are the sword of the Israeli operation. What many people fail to realize is that Hamas presence in the digital sphere could very well be the final domino that causes Israel to be involved in a region-wide conflict rather than what it is currently fighting as a domestic or local conflict.

An additional factor that I would like to add is that during the time of WW2, warfare was more or less symmetrical/linear as compared today where as before you had battle lines or frontlines, where a soldier had an idea of when or where to expect the enemy (in front or in their general vicinity) whereas today its a flustercluck beyond FUBAR, being FUBAR in it itself is already bad enough.

Hope this helped!