If you mean 24s by "small" then note that you can fit two 6s where the 24s will go as long as you have room above for the somewhat taller 6s.
Two 6s in series gets you 232AH (they vary between 208-232 roughly) while your two 24s (85AH each?) in parallel get maybe 170AH.
There are a million threads on 12s vs 6s but there is no dispute about the AHs you can get either way. Of course you can get a pair of bigger 12s to match the AH of a pair of 6s--but in your case you can't fit them on the tongue. Although others have made bigger trays on their tongues to hold more of a battery bank.
