After a good workout or in the time of workouts my muscles feel big and feel like they are going to explode out of my skin why does that happen?pumped

Vascular dilation: Your muscles get engorged with blood which makes them larger. This is the natural response when muscles are being used and demand more blood--therefore they temporarily grow. Over time (long term) they increase in size because of muscle fiber hypertrophy. This is what makes them larger and stronger.

Increased blood supp: After a rigorous work out, there is increased blood supply to the muscles creating a feeling of pumped, full feeling and sometimes feeling of pulsating.