[PlasmaKrab]

As far as I know, the BMP and BTR units had similar operational concepts, in other words, both vehicles could have been swapped in mechanized units without modifying the squads.
The BTRs were more mobile, cheaper and had more amphibious capacity.

I've just checked the default game OOBs, and it turns out that both BTRs and BMPs have a carry capacity of 109, which is identical if optimistic.
Though, the BMPs all have a crew of 2 (driver and gunner), which reflects the fact that the squad leader/ vehicle commander is supposed to dismount with the squad. I don't know if that is coherent with Russian doctrine, but it makes sense as far as the in-game formations are concerned (platoon leader is always an infantry unit).
In comparison, the BTRs have a crew of 3, regardless of a lighter weapon system, which means that the commander stays on board for some reason, and that in theory, the vehicle carries one person more.

As to the Vodnik having a capacity of 6 men, I couldn't comment on that. Maybe it's supposed to reflect the reduced capacity of a vehicle beefed up for airdrop and supporting a para section? Or to keep in tune with the BMDs?
These vehicles are modular, so you could probably go for larger troop-carrier boxes and reach the BTR-Ds 110.