Answer:
Your lips get bigger because they swell up a little, and this is perfectly normal. It’s a sign that you are aroused and like his touch. It’s a good thing, trust me. It could hurt because of various reasons. Make sure that his fingers are cut short, so he can’t scratch you inside. Also, maybe he goes to deep or does it too roughly. Another possibility is that you aren’t wet enough from the beginning. Solutions? Make him go slow and built it up, if he is doing it too roughly, tell him. Put your hand over his as a signal to go slower or less deep. If you aren’t wet enough from the start, then take your time. Explore each other’s bodies. Make him kiss you all over and touch every piece of skin. You have to be fully turned on before doing stuff together, okay?
The hurting will stop eventually, don’t worry. You both have to learn how you like to be touched and how to touch that other person to make them aroused and ready. It’s normal, so don’t be afraid.
The swelling however, will last. It’s a sign from your body to you and your partner that you enjoying it and are ready for more.