OK, an update from the OP. I guess I should have done more of my own research first... sorry!

Turns out the trunks are roughly as follows:
Volvo C70: 32" x 38" x 12" High
Mustang: 23" x 45" x 15" High

So the Mustang is ever so slightly more cubic space. I actually went to a Volvo dealership today and looked at a C70 - it's an interesting design, the hard top flips into the trunk, filling the space on top and the "trunk space" is underneath. The BIG difference is that with mid-sized checked bags, typically 26-30" high, you CANNOT get 2 into the Mustang because the trunk is too short (front to back).

SO - bottom line, I can get 2 checked bags (18" x 27") into the C70 but NOT into the Mustang.

If any of the previous posters have facts to support their data to the contrary, I'd love to hear it. Maybe Hertz has a special C70 model built just for them...