There's nothing to trim from PS1 ISOs, The ISO size is the same as the Game's data size.
Trimming is used only on Read Only Memory chipset dump (ROM), because when you dump a chipset you read all "sectors" of the chipset from start to end, whether there are data or not.
Then, trimming means removing the ending part of the resulted dump file where there's no Data (the part after the game's size up to the chipset size, which could be up to 49% removed from the dump).
Chipset are always multiple of 2, so the chipset size are 2, 4, 8, 16, (24 too), 32 etc.
Let's talk in Megabit (chipsets size are always counted in bit not bytes)
If your game is 5Mb, it will not fit in the 4Mb chipset, the manufacturer will use a 8Mb chipset for only 5Mb of game's data.
When you dump the game, you end with a 8Mb file dump, but only 5Mb is used so you can safely "trim" the 3Mb of unused data from that dump, which is (always?) located at the end of the chipset.
If you want to reduce an ISO size, you need to edit the game's content and remove data (like unused languages if the game is multi-5, replace all videos with a dummy/shorter videos, etc.)
PS1 data is not encrypted or signed, you can edit and replace the files with a simple file explorer and make a new ISO.
If you want to reduce the file size for storage purpose only, you should check ecm tool from Neil Corlett :
http://www.neillcorlett.com/ecm/