I've had adult (so yes, sexually mature) axies that were about 8-9inches when I got them that still grew several inches after. So yes, I believe they do keep growing after they are sexually mature, but how much bigger they will grow is harder to determine
From the Axolotl Biology page:
I once saw a friend's female axolotl that was about 43 cm (17 inches) in length. However, a large size for most axolotls would be 25-30 cm (10-12 inches). The average seems to be about 23-25 cm (9-10 inches). Adults tend to reach their full size after eighteen months to two years, the growth rate depending on how well they are fed and at what temperature they are kept.
I'd say your axie still has some growing to do