Headphone Jack too small for 3.5mm

How can a “3.5mm” jack have different sizes?
Either it is 3.5mm in diameter, or it is 2.5mm, isn’t it?
Or dou you mean the length? But also the length should not differ, because it is standarized (and mono, stereo, headset just differ in the number of rings, not in length, afaik.)

A photo would be really helpful (one of the jack, one with jack inserted in the FP2).
Maybe you only mean, that the jack is just 1-2 mm to LONG (not to wide), as you can see at this pic (from this post)?

The left side of the image are taken WITHOUT the case, the right side is with case.
As you can see, the 3.5mm-jack sticks out in both cases for some mm - that seems to be normal.
In this case, everything is correct, thats it is standard with FP2!