Yeah, I wouldn't bother unless it's significantly smaller. And by the looks of things some of the 15 watt amps are quite a bit smaller. I could be wrong though. That's just based on some videos I've seen.
All I know is that the Rumble 40 (although an awesome amp) is quite a large obstacle in our small living room (especially with a 2 year old running around) so I usually keep it hidden away in the only spare corner.
You're right about the volume thing. I could turn the amp down. I just felt it unnecessary to have an amp of that size and power when I never get to utilise it.
I figured a smaller amp would solve those issues, but if that's not the case, I've got some rethinking to do...