Konsole always opens maximized, regardless of default size settings in Konsole settings or even KWin settings. Forcing the window size has been the only way I can get Konsole to not open maximized.
Does this reproduce with a new user account?
No, it does not happen on a different user account
Then it's a configuration issue with your user account and not a bung in Konsole. Perhaps there's a KWin rule that's forcing it to be maximized, for example.
I've triple checked my KWin rules and my Konsole configurations. Nothing should be causing this behavior.