"I always thought there may have been more rebel bases out there aside from Hoth (which would explain why in ROTJ the rebellion suddenly has more than just humans in it). If this were the case, then the Empire's invasion likely was just a fraction of the power that they could have sent in.

Or I'm wrong."

I'd say you're right oh-riginal. I too believe their invasion was just a fraction of what they could have sent. From what we can see, only 3 SDs (maybe a few more off-screen) plus the Executor were sent, but they definitely have more than that somewhere else.