Painkillers like ibuprofen are NSAIDs which inhibit the enzyme COX1/2, reducing prostaglandin production.

Prostaglandins are an inflammatory hormone that do a variety of things, but specifically PGE2 plays a role in muscle stem cell activation to divide and produce more muscle fibers. The effect is probably realistically small, but you will leave gains on the table by taking ibuprofen after hard workouts.

Are you saying that lifting weights makes more muscle fibers? I was under the impression it does not, that it simply makes your existing muscle fibers bigger and stronger.