Some missions can have low ammunition as a descriptor. Also low ammunition is on the random complications table in the GM section.
I would assume they mean virtually unlimited normal ammo, but reloads are required. I would assume each weapon could be reloaded at least three (3) times.
Examples:
A) A bolter and nine (9) magazines would be standard (three are loaded at a time).
B) A bolt pistol with thee (3) magazines for all but an Assault Marine would have twelve (12) magazines, perhaps twenty-four (24) magazines. Remember bolt pistol clips hold half the bolts compared to a bolter clip.
C) A heavy bolter would have the backpack with 250 rounds and three (3) 60-round clips.
). I'd look at the HB as more of the guy toting around the .50 cal or a minigun than a guy with the SAW- he can't move and fire (though he doesn't have to brace), he's not part of the rapid assault force in the squad. His purpose is to do damage to light armored vehicles and rake enemy formations with death, in addition to suppresion. While space marines are hulking and 7ish feet tall and don't usually need to worry about weight, you still have volume to consider- they don't exactly wear web gear and have ample pockets, so all that ammo has to sit somewhere.