however when you start your car engine then you are delivering considerably more than 12v of power. It is this additional power which can cause damage to the motorcycle.
Additional power, err cranking amps ? the system draws just as much as it needs, and no more...jump start of a U boat battery pack if you want, just as long as it is 12 volts.
Type 21 U boat 34,000 Amp hours capacity, Kz bike 8--14 amp hours.
Oversupply of 'power' or more precisely cold cranking current is never a problem, it usually the other way around when the battery is tired and the connectors bad: battery can't supply enough cranking current so voltage drops...hence the ballast resistor approach on some kz.
I've jumped off running motors on bikes and cars for decades, breakdown recovery drivers do it all the time.
OP A badly defective battery will never hold charge and wen really bad acts like a big resistor and just pulls the sytem voltage down. Go through your charging system starting with battery, battery leads and earth straps and check you alternator stator connections etc.