Battery size per-se does not mean much to your charging system.  You have excess electrical alternator output to charge the battery, or you don't.

I have a riding lawnmower with a full size auto battery on it and the puny Briggs and Straton flywheel magneto keeps it charged up just dandy.  I have endless cranking power on tap, even when the engine is hot.  Crank time is a function of battery size.  

Charging is a function of running load and alternator output, with all the excess alternator output past the steady load being available to charge the battery.

What has to be a match is the running load (running lights, heaters, horns, stereos, headlight) and the total output of the alternator/rectifier system.  There must be an excess to go towards charging the battery.  

You can see this excess on your volt-ohm meter in DC volt mode with probes on the battery contacts, read the engine off steady voltage level of the battery (at least 12.5 volts) and this should go up to 14-17 volts when you start the engine and rev it to quarter speed.  The difference between the rev reading and the steady state of the battery reading is the voltage and amperage you have available to charge the battery.  You should do this test with all the normal loads like headlights, running lights, stereo etc. in play to see if you really have something left to charge the battery with.

You can lose alternator output to corroded or loose connections.  One connection everyone forgets to check is the main ground connection to the engine and to the frame.  Corrosion here will make your whole electrical system (including the firing of the ignition system) suck.

Other ways to lose alternator system output is to have a weak or bad rectifier, or to have lost some coil windings in your stator.  A volt-ohm meter check on some specific wiring connector pins can answer this question for you without tearing down your engine.

Hidden loads (such as mild shorts through worn wire insulation and corroded connections under bike or in headlight or a badly corroded fuse box) can push up your load balance to teeter in the wrong direction.