Your muscles may swell for a few hours or days after a workout
This is a good question, as you will likely have been told that it takes time for your muscles to get bigger. So why does it look like they are bigger almost instantly? You’ll be happy to know that it isn’t in your head. However, what you are seeing isn’t a growth in your muscles, but a swelling in the area as blood flow increases. When you exercise, your muscles need more oxygen, and so your blood vessels will dilate near your muscles. It’s this that makes your muscles temporarily larger.