Well how long have you had she/he? If you want to know if he is a he or a she you can try holding (it) in your hand and either turning it over (which they sometimes don't like) or holding it above you head,, now what you are looking for is, V shaped pores (or dots) on the belly area right before his tail, now depending on how old she/he is he may not have them yet, but if his tail is real fat it should be old enough to sex. Now if you don't see anything chances are it is a she.
Can you take pics of the Leo so I can see the size of it's tail?
Back to the feeding: I'm sure you are feeding crickets right? Well if you are concerned with how much (it) is eating you can get some tweezers or tongs from the
pet store, they are called 'Feeding Tongs' and start hand feeding the crickets to (it). Also if to many crickets are in the cage and they aren't getting eaten, the cricketscan start to stress out the Leopard Gecko and they also will start eating on the gecko!
If you don't feel comfortable hand feeding you could also just drop one at a time in there and if it doesn't get eaten then don't put anymore in there. Also don't offer any food for 2 or 3 days then feed them and wait another 2-3 days, I find it is less of a struggle if my Leos are good and hungry instead of just a little hungry that way they don't waste as much food and I don't sit there for 20-30 minutes just trying to get them to eat.
Hope tthis helps
and this is just things that work for me, and if it doesn't work for you then maybe along the way you will find a way that works really good for you and your Leos.
Oh and just to answer your question about it's tail being 'Too Big?'. There is no such thing! In fact some breeders or collectors just look at tails, it's like their health meter.
A big fat tail is an awesome sign.