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

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.
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.