Another thing that might be throwing out the sense of scale is the height of the walls around the pools and their thickness.

I notice that you have openings in the wall around the structure (presumably arches to allow human access). Arches like that tend to be about 10 feet wide and somewhat higher than that (say 12-15 feet). At least, that's what my "sense of scale" tells me.
If the above is true, then the walls around the pools are extremely tall and wide. Nobody would be able to see into/over them. And, since they are (in your original pic) one of the 1st or 2nd things that get looked at, the mind sees them, adjusts itself to their size (and assumes the height of the pool walls to be, say, 1 foot) and then looks at the bulding again - which now appears to be 8-12 feet tall...

Just my 2-pennyworth..

Cheers,
Diolma