I have big hips and thighs, but a smaller waist, so I get that awful gap at the top of a lot of my pants. I like wearing belts as an accessory, but I don't like that I HAVE to wear a belt so my underwear (or more :-/) doesn't show. This gap is especially bad on shorts because shorts are super casual and I never want to wear a bulky belt with them. So I took pictures to share how I fixed this problem on these shorts.
Before: stupid gap!
Cut about a 6 inch piece of elastic and pin to the back inside of the waistband.
Stretch the elastic as you pin it, so when it's unstreteched the waistband is sort of gathered.
Sew two lines with a large zig-zag stitch, stretching the elastic as you go.
If you use matching thread it shouldn't so very much on the outside. I just sewed right over the belt loops because I never want to wear a belt with shorts, but if you want the option you'll need to stitch it in two parts, one on each side of the belt loop to keep the loop free.
This is what the inside looks like. I used black elastic since my shorts are black, but it shouldn't show in the end so it really doesn't matter if you use white elastic. If you wanted to be super fancy you could encase the elastic in matching fabric or something to make it look more finished.
Ta-da! No more gap!
And now the internet knows just how big my butt is.. :-/