Don't batteries usually just give you the Ampere-Hour rating (Ah)? A 12V battery with a 80 Ah rating will give you 80 hours at 1 amp or 40 hours at 2 amps etc.
A bigger capacity battery may not give you any higher maximum current capacity but will crank the same motor over for longer. The current is determined by the load (i.e. starter motor) subject to the battery being physically capable of delivering that current. If the current capacity of the battery is low it might not turn the motor as quickly as a high capacity battery.
I now only use the Optima spiral cell type, which seems to give about 20% higher max. current capacity, so it will crank an engine over a lot quicker which can make the difference between starting or not, especially with diesels! Because a low current capacity battery will crank a motor slower than a high capacity one will.
My GMC and Jeep 6V batteries are only 50Ah but will spin the engines over much quicker than a heavy duty 6V conventional battery of greater Ah capacity. Another advantage is you get less voltage drop - my jimmy used to have only 3V at the coil when cranking :shake: Mind you, it still started :-D And the 6V ones are only 1/2 the size of a standard 6V battery!
For heavy duty 24V applications, using two 56 Ah 12V batteries (112Ah total) to replace two conventional 140 Ah ones (280Ah total) still gives much faster cranking speed (if the engine starts instantly you dont need the extra Ah 'cos you are not cranking the engine for anywhere near the same length of time, so you never drain them). But they do come at a price.
Back to your problem, sounds like the batteries are simply tired, and need changing. With batteries you usually get what you pay for - a cheap battery will tend to have less lead and often of a much lower quality, so may last little time compared to something heavy and expensive :cool2:
Sorry just changed some of this 'cos it was scribble - hope it makes more sense now!