Let's say I'm doing french-press: https://www.youtube.com/watch?v=pQJl5yYOEbE with two options. Once with wide grip, and elbows kept wide. Next time close grip, elbows - the same. Other parts of move - the same. I mean head touched at the same point, same about end of move. Question is about load I can take in short grip, or better to say it's drop. Why is that narrow grip gives big drop at load? I can expect that it makes long head playing main role, while wide grip activates side heads. Can anyone explain that in details?
Correct me if I'm wrong, but you want to understand why you can lift heavier with a wider grip?
In this isolation exercise, I think the grip is not the most important : it is the elbow stance that plays the most part. Indeed, the wider the elbows, the easier it is.
You can try doing dips with elbows close to your body (i.e. parallel to the bars), it will be more difficult than with your elbows open.