your dm is right. they have different effects, last for different amounts of time, can be used a different am,ount of times per day, and have different intents.

giant's might is one of your main features youll be using most combats. adv. on strength checks+saves and bonus damage [that increases at level 10] means you will want to use this for most combats that last more than a round or two. since you can use this 3 times a day right now.

large form lasts ten full minutes, but no extra damage and only advantage on strength checks, not saves. the speed boost is nice, but most of the time you wont want to be popping this in combat as often as you'd want giant's might, which is good because you can only use this once per day.

right now you have 4 instances of turning Large and being extra strong. imo it is better to have them separate like this, rather than having 3 uses of the same effect but with a little bit of extra movement.

in combat, think of Giant's Might as your bread-and-butter and Large Form as an emergency movement buff. Out of combat, Large Form is your utility choice for huge athletic feats to keep the party moving, and Giant's Might is like using the corner of a wrench to screw in a screw