I just did this for the first time yesterday too! LOL! My "uniform"-ish t-shirt I wear on Thursdays is too big and yesterday I thought I'd try tucking it. I don't like the way pants pucker out under the belly button and I felt like my pants were doing that yesterday, even though it was just because the pants are sloppy. They're a little too big too.
Because the pants AND shirt were too big, I felt like there was extra air in the material.
Anyway...DH immediately commented on how thin I looked!
So it must work despite my own perception.