"Does every trimmer 'trim' the same same amount? Are the differences that some have wifi detection and some don't? (apart from batch mode etc.)"
You should look on Google for rom trimming. There's a lot of information out there.
Basically, each game is a different size. They're limited to specific size increments due to the file system I suppose, they are 2MB, 4MB, 8MB, 16MB, 32MB, 64MB, 128MB, and so on….
If a game is, for example, only 99MB, then the game needs to be patched with useless code to make up the remaining 29MB that it will fit perfectly on a 128MB cart. That useless code simply takes up space, and that's it.
A rom trimmer removes the useless code. To put things simply, there are two types of rom trimmers, "agressive" and "safe"
Safe trimmers take off some of the redundant code (the useless code), but they are more careful to ensure nothing it removed that shouldn't be.
Agressive trimmers take off a lot more of the redundant code but they're not as 'careful' in their approach, and sometimes they can accidentally remove the code that is needed to support WiFi features.
Like I said, google, google, google. There's a ton of information out there on this.