Contraction means that it gets shorter, and as it gets shorter it also gets thicker.
It morphs into a diaper.
When the biceps contract the triceps relaxes. It happens when your arm is bent.
When triceps are in contraction (flexion), automatically biceps should be in extension. Biceps and triceps are antagonist in function.
When the bicep contracts the triceps relax
A muscle that contracts shortens whereas a muscle that relaxes lengthens.