I understand that rom trimmers reduce the file size of the nds file, but my question is do they delete for example, the online portions of the game. I saw a thread saying that it is possible they may delete wifi portions and so on (online play). Is this true or has anyone experienced this?
Old trimming programs sometimes did delete certain aspects that allowed for multiplayer, any up-to-date trimmer does not do this, some even give you the option to remove the multiplayer data should you not use it to reduce the filesize even further. The amount of space saved is depends on the game, with larger games you often remove more (but not always). DS roms are always a certain size out of these: 8MB, 16MB, 32MB, 64MB, 128MB, 256MB. If a game is say 24MB big it then gets padded upto the next size level (in this case 32MB) the rom trimmers simply remove that.
IMO, trimmers delete dummy files found inside the rom. I mean, it's practically impossible to fill exactly 8/16/32/64/128 megabytes with script, sound and graphics. Therefore, developers put a lot of meaningless files that take up the needed space. And it seems that saves take up exactly 512kb
