UPX only means smaller files on the disk, but it comes with a cost: it tends to increase memory requirements, because the binary on the disk cannot be mapped to memory anymore. Unless it's uncompressed somewhere in the filesystem.

Worse, if you run multiple instances of the same binary, none of them can be shared.

A bit simplified, without UPX, 100 processes of 100 MB binaries requires only 100 MB RAM for the code, but with UPX 10 GB.

Edit: In reality, likely only a fraction of that 100 MB needs actually to be mapped into memory, so without UPX true memory consumption is even less than 100 MB.