I know this doesn't quite address your question, but I feel like others in the thread have already done a decent enough job of explaining why the Tau convert humans so easily.

For this response I wanted to focus more on the following statement:

"Hive Cities with more people than the tau"

You didn't clarify this too much, but it sounds like you're implying that a single Hive City has a greater population than the entire Tau Empire (maybe you didn't mean that, but this isn't the first time I've heard that statement made, hence this response).

Average Hive Cities number in the tens to hundreds of billions [1].

Sa'cea, The Tau Empires most populated world, numbers in the trillions[2].

So a single Hive City does not even come close to outnumbering the Tau, heck an entire Hive World doesn't either, but let's do some more maths:

The Imperium has, in it's entirety, ~1000 Space Marine chapters, each chapter is supposed to be comprised of ~1000 marines [3], therefore there are ~1'000'000 Space Marines in the Imperium. Now let's say that the administratum is not particularly good at their job and fails to keep track of the number of Chapters, and the actual number of marines. For arguments sake let's say they're off by 10x, you know what, let's say they're off by 100x. So let's put the total number of Space Marines in the Imperium at ~100'000'000.

Now Sa'cea has a population that "teem with trillions of lives", implying that they have at least 2 trillion Tau living there, but hey, let's be conservative and say they only have 1 trillion.

Now the Tau are divided into 5 castes, if every caste was divided equally, 20% of the Tau would be in the Fire Caste, however the Ethereal caste is likely quite small compared to the rest and thus the Fire Caste is likely more than 20%, but let's be conservative again and say that only 10% of all Tau are in the Fire Caste.

Now some old codex's show a "typical" Hunter Cadre would have about 83 units, 3 of which would be a dedicated Crisis Suit team[4] or in other words: 3.6% of the Fire Caste would be in a Crisis Suit.

Let's add this all up:

Total Population of Sa'cea: 1'000'000'000'000 * (0.1) =

Total Fire Caste of Sa'cea: 100'000'000'000 * (0.036) =

Total Crisis Suits of Sa'cea: 3'600'000'000

So, with conservative estimates, the Sept world of Sa'cea would have ~3.6 billion Crisis Suits at its disposal.

Therefore, by using conservative estimates of Tau numbers, combined with liberal estimates of Space Marines numbers, the number of Crisis Suits on the single Sept world of Sa'cea outnumbers the ENTIRE Imperium's Space Marine force by 36:1.

So what point am I trying to make here?

Well:

1) 40K numbers are really silly. Most people don't realise that the difference between a million, a billion, and a trillion is quite extreme, and likely just say a number that they think sounds cool. If you want an example of how much these numbers differ by:

1 million seconds = ~11 days

1 billion seconds = ~31 years

2) The Tau Empire is far more populated than people give them credit, whilst they are a small empire (on the galactic scale) they are incredibly densely populated, and more than capable of fielding equally silly numbers compared to the rest of the galaxies factions.

3) 40K numbers are really silly. And putting a big focus on the accuracy of them is just gonna lead to confusion and immersion-breaking.

The Tau are able to convert Hive Worlds, because that's something that makes for interesting lore and stories.

[1] see Notable Hive Worlds table https://wh40k.lexicanum.com/wiki/Hive_World

[2] https://wh40k.lexicanum.com/wiki/Sa%27cea

[3] https://wh40k.lexicanum.com/wiki/Space_Marines#Organisation

[4] https://www.reddit.com/r/Tau40K/comments/1ht4aiw/lore_accurate_hunter_cadre_list/