depends on how efficiently you mount it. I assume this is sidemount.
canister is under the crotch strap, have all of my spools/reels back there *typically 4-6* and my clip with cookies/arrows. No issues with any of that back there. The primary reels get a little awkward but that's only because they are big and heavy. Everything else is small enough that it isn't a problem.
Stages can also get butt mounted after they're empty, extra scooter will go back there *though that's on the front crotch d-ring, but it ends up back there*, not a whole lot else to put back there. The Razor guys will put their purse back there, but I prefer a thigh pocket for things like extra double enders, wet notes etc