I'm sure Disney is well, well aware of the enormous number of strollers and
ECVs on their property every day. So I would imagine that they would not work out numbers based on there not being any strollers or ECVs in the gondolas. I also doubt they worked their numbers on 10 people per car even if there are no strollers or ECVs, because as I mentioned before, Disney is also aware that some of us guests are much larger than others.
Honestly, in terms of transportation function, I don't think it really matters how many people get on each car. The beauty of the gondola system is that there is a never ending supply of empty cars moving into the station all day long. So even at the busiest times, you'll never have waiting times when absolutely no one is getting into a gondola car like you do with other modes of transportation. It's not "Oh man, there went the boat. Now we have to wait X minutes for the next one." or "Ugh! This bus is full??? Now we have to wait for another bus??". With gondolas that doesn't happen. This car is full? No problem. That car just left? No problem! Because there is a constant line of empty cars pulling into the station.
Heck, it may be easier loading small groups than worrying about filling the cars completely, anyway. Less jockeying for position between groups/couples/etc. who don't know one another.