just about everything.

full frame sensors produce images with higher image quality

But why ?

lower noise

That might be, but why ?

wider dynamic range and/or higher resolution

If the pixcel numbers are the same and the aspect ratios are the same,
then the resolutions are equal. My question assumes shorter lenses to
yield the same image. Nothing cropped.

and the lenses aren't necessarily bigger.

Not true, under the above premise.