I'm sure it's no surprise to you that growing out young 2.5" fish in a planted set-up, no matter how attentive you might be to doing large, frequent water changes and a good tank cleansing routine, generally results in appreciably slower growth than what would normally have taken place in a bare-bottom set-up, and lesser than optimum growth into maturity.
As you recognize, that has indeed been the result in your case, but it's a credit to you that you have nonetheless managed to do an excellent job of maintaining a smaller than usually recommended group into a seemingly very good looking and healthy condition from the outset - you are to be congratulated for having accomplished that.
Many fail miserably attempting a planted tank grow-out of very young discus as you likely well know.