All Amigas could have up to 2MB chipmem, above that, up to 512MB fastmem is possible. For A500 (only) there was an expansion of up to 1.8MB on top of this, called "slowmem" or "fake fastmem" because it didn't give the 33% speed boost, the reason being that it was connected to the same local bus as the chips.
Blitter setup only takes a few cycles, and it can be done with the copper or the CPU, no difference and not a factor for speed.
Most games shrunk the screen buffer to make the polygons tiny, so that the render time would be small for the CPU. Then it would be small for the Blitter also, and the calculation overhead for each small polygon would be the same. Many demos have larger polygons on full screens, running at fast framerates, and also use features of the Blitter with more insight than games did.