Does anyone know if these are the same? It seems that sucre glace is more dense than powdered sugar and when I made a cake and substituted sucre glace for the powdered sugar it was way to sweet. Is sucre glace a good substitute for powdered sugar, or should I be using something else?

it's the same thing.

Are you adding it to the cake mix?

no I was trying to make a gooey butter cake and it calls for 3.5 cups of powdered sugar mixed into cream cheese/egg mixture. I added 3.5 cups of sucre glace and it seemed like way too much sugar. The powdered sugar seems to be more fluffy and the sucre glace seems heavier and thus maybe I should have used less...

It probably just packed down. This is why volume measures are not good for baking. You probably need to sieve it before you measure.