E-7 or change to m4/3

The "ginormous" version has much sturdier build quality, is optically better, has smoother bokeh, better contrast and micro-contrast, and would cover a larger sensor (if one existed). It's easy to build smaller lenses if you compromise the materials and optics.

For a different comparison, the 45mm and 50mm are somewhat similar in size; the difference is about 15mm in both width and length. Optically, even the 45mm isn't quite up to par with the 50, which is also a macro lens with weather sealing. The 75mm could optically be about the same - and it would also be the same size in 4/3 mount.

You win in size (for shorter lenses), you lose in optical quality. That's how it goes. TANSTAAFL and all that.