There's a battery FAQ ....and I believe that it speaks to the capacity of different sized connectors. Basically...for mini batteries, mini connectors are fine...for larger batteries, use larger ones.
Manufacturers use mini connectors on everything because they are cheap.
Put a big battery through an upgraded gun and you'll be beyond the capacity of the small connector. Causes heat. The way Tamiya plugs get loosened up over time, I've seen guns with melted plugs.
Switch to large plugs...or DEANs. You can make an adaptor for your charger. You really should get a decent charger if you want you batteries to perform at the max potential.
Or...don't sweat it and just replace things as you need to.