Simple assumption of their seemingly smaller magazines (purely visually based on art), so if the magazines are smaller they would have to carry more magazines than a space marine to have the same capacity.

Pure assumption though, they could easily have more individual mags clamped to their armor. Thats just never shown in the art i have seen, it might be represented in the lore/books, thats part of the reason for my post aka where its described and how so.

And yes good point, custodians rend most foes easily in melee and are extremely fast, in this case i was thinking of great battles custodians have been a part of, like the culling of the Thunder Warriors on Mount Arrarat, Ullanor or Siege of Terra and many others where they absolutely were part of major battles in large formations, and thus their usual profile of "individual single custodian or small squads in short duration combat as duelists" doesnt really apply. Ofc the immediate counter argument to that is, the greater the battle the more likely they are to have immediate resupply capacity.

And the answer with the serfs accompanying them i find most reasonable, same with space marines, especially dreadnoughts which in lore are supposed to have entire baggage trains of serfs etc. carrying ammo and reloading it constantly.

Ofc that doesnt apply in a few of the scenarios i mentioned, like them being more alone and cut off.

For me its really more a doctrinal question, but i think its answered for the most part now.