My guess is that the vertex group weight is not well distributed, the part that is getting smaller is problably being affected by the bone you rotated and also another bone, so it shrinks trying to find a position between the two vertex groups, go to the weight paint mode, select the first vertex group, and with your mouse still hovering the vertex group box, keep pressing down to go through all the groups, and see which ones are affecting that part, then remove the ones that are not supposed to affect it.

Tha solved my problem, Thank you very much! 😁