it's a pounds per sq. inch thing for scrubbing power, so a smaller diameter ie: 15" with a 1.5hp motor can very well be a good choice in residential. All the 13" I've seen are fairly light, but if you can find an oldie it might have good ratios and serve quite well.
My thoughts on scrubbers in general is; the one to use is the one you are actually likely to use, so the 120lb monster may not be a reality for you.
Don't under estimate the GLS or Host type counter rotating brush machines, and they are lighter.