Judging by the height of the visible portion of the trough on the far left I'd say you only have 10-12 inches showing where the cat is sitting. Making the cat roughly 12-14 inches tall when standing on all fours. Not really a big cat at all.
Not correct. I printed out the photo of the cat looking straight at me. I then drew a straight line across the top of the back of the trough. Then, I measured from the lowest part of the trough I could see with no grass and made another line from there parallel with the first line. Then I made a grid. Although this first attempt is pretty rough, I know the trough is 21 inches high (I mistyped before and said it was 20 inches high) and I think the grid would be 18 inches square but I'm willing to give away 2 more inches and call it 16 inches square.
The cat is JUST under two grids making it around 30 inches tall in the seated position. Think about how cats walk. Where is their backline? Just above the top of the shoulder. In the photo, where is the shoulder? Just under the chin. That would be 16" plus about 6". That's 22" at its backline. I know this is all just approximations but I think they are conservative numbers.View attachment 121403
I think the flaw in you measurement is you're starting in the middle of the trough and thinking that you can see the full 21". You can't. The far left shows much more of the trough than in the middle. Assuming the wall is 21" all the way across then the far right shows only about half of the height. which would be 10"-12". Additionally, the grid line that you drew at the top of the trough is at the top of the FAR side, not the near side. The parallax is making your grid taller than it should be. I still say the cat is about 12"-14" at its back based on the reference that you drew.
I'm really not trying to rain on your parade. Just based on the visual references provided the cat is normal size.