I saw there is a new (patch) release upp-win-16271.7z on the homepage.
I exract this new version by overwriting the old one but got same problem.
So I deleted upp whole directory and started from scratch. This time the IDE works normally in that it consumes only around 40 to 70MB memory. That's really weird. I did not remember what I did with the old version.
However, I got a new problem. I cannot build a minimal U++ console app using 32-bit compilers. I remember I could do it with 16270 version.