I have 3 cats, 10.5lb, 11.5lb and 12.5lb but the 12.5lb used to be 18.5lbs. These cat tree's still worked for her.

Yaheetech. Top pedestal is bigger than most as well as top laying section. And the house is surprisingly big, even when my girl was 18.5lb she fit in it. She also LOVES the scoop. No one used the hammock, so detached it and the ramp cuz it's stupid.

Go Pet Club . My former chonker could not fit her entire body in the top house, but she would still go inside it with half her front or back laying outside. The top pedestals are not the roomiest, but she would also sleep in them, with her extra bits just hanging over the edge. I took the ramps off this guy too.

Edit: if you're going for something slightly cuter, I also have this. My guys all love the top pedestals. The boxes are on the smaller side. My former chonker did not fit at 18.5lbs but does now at 12.5lb. It is also on the shorter side. When we had the other 2 taller ones and this guy all competing in the same area, it was definitely the loser. But now that it's upstairs in front of a window, it's the bedtime hang out spot.