Batteries are rated in Ampere/hour - i.e. the maximum amount of current available for a given time.

Are you confusing 50AH and 100AH with pure current load?

In general use we never put mismatched batteries i.e. with different AH rating in parallel.

Batteries conform with the rules of physics. i.e. the bigger the battery - (the larger plates) will give a larger capacity (measured in Ampere hours)