I made a dolly out of 1X3s, added 2 1X3 pieces to fill the gaps and screwed a 3/4" plywood platform over the entire thing before bolting the bottom section to it.

So, yes, the legs are bolted to/through the plywood.

I just measured it: 25x30" with 4" wheels rated to hold 300lbs (I believe, each). I haven't weighed it but it is hefty with the bottom being mounted to it.