The batteries have an internal resistance so the voltage drops when the load current is high.
Alkaline batteries:
An AAAA might produce only 1V at about 400mA for a few moments.
An AA might produce only 1V at about 2A for a few moments.
A 9V has six AAAA cells inside so it might produce only 6V at about 400mA for a few moments.
A C might produce only 1V at about 4A for a few moments.
A D might produce only 1V at about 10A for a few moments.
The datasheets show graphs of how much time a battery can power a certain current and you select the current and the voltage at the end.