Amphs started small, and wattage grew along with venue size for major acts. PA systems got louder, too, and folks started realizing that you could mike the amphs and keep stage volume to a minimum.

Jimi Hendrix had a couple of full size Marshall stacks on stage, and soon everyone wanted to look cool (like him) with those big stacks.

I remember in Austin in the early 90s EVERYONE that played rock had a half stack, even in some tiny clubs.