It depends on which master's skill you equip. Master's skills allow you to fight more or less exactly like your chosen master.
So for example you are using your medium size character and you equip your Bardock master skill. Bardock has a very power based fighting style but his movement tends to be really slow so unless you are good enough to overcome it that loss of speed could end up being a huge burden to you.
In contrast, if you were to equip a very quick, Super Gogeta master's skill your character would be very balanced and at the same time very quick which is why most people tend to equip his master's skill.

Well ,like cutekitten78, it depends on which master you use, but I may say it could make your hero weak.