[JRTJH]

I'd suspect the biggest problem with two batteries of different capacity would be in charging them. One battery will be "fully charged" while the other would be "still needing charged". So, if the battery charger shuts off, you'll never fully charge the larger battery. However, what usually happens is the larger battery still needs charging, the converter senses this and keeps sending "top-off" current to both batteries. This essentially "overcharges" or "burns up" the smaller of the two batteries since it can't keep absorbing power while the larger battery keeps accepting the charge "happy as could be"....

When you stop charging, the smaller battery depletes faster, would be "0 left" while the larger battery may still be 30% charged and trying to "push power" through the discharged smaller battery. That would stress the big one and cause the smaller battery to be damaged with repeated recharges.

Neither condition, discharging or recharging would be "equal" for the batteries and one is "always stressed".... It's no wonder one "gave up the ghost" while the other one is probably getting close. As to leaking battery fluid ??? Was the case damaged or was there some other reason? It's probably not related to the size difference unless it was at the caps/vents which could be indicative of an overcharged battery that's getting too hot and vaporizing the fluid. That would probably be the smaller of the two, if that were the situation....

Of course that's only a guess, but having two different size batteries is not a good idea, in fact, it's probably a very bad idea, especially for battery reliability/longevity.....