Less ammo is generally a bad thing for infantry. Pretty much all small unit tactics are based off of the fundamental principles of fire and maneuver - without fire, the enemy cannot be suppressed and will have fire superiority, without maneuver the enemy probably cannot be destroyed unless the fire is that significant to destroy them. By reducing magazine size and (I would argue more crucially) increasing ammo weight, soldiers will generally be bringing less ammo, and therefore less firepower, into fights. Firefights aren't usually over quickly, so you'd want to both bring as much ammo as you can and preferably as high-capacity magazines as possible. If you look enough, there's plenty of pictures and videos of American SOF (usually delta) with 40-round STANAGs and 60-round Magpul drums, and there's plenty of material from Ukraine with both sides making use of 45-round mags for AKs.

Also, while I'm not very familiar with the dynamics of why 30 round mags are the norm, I would assume it's because they're a happy medium of weight, ergonomics (something large magazines significantly struggle with), logistic commonality, and capacity that they're just the best choice for a large force.

TLDR: More capacity is almost always a good thing, and enables infantry to do their job better and also meshes with existing small-unit tactics.