>>247124
>But I don't want big legs.
Sorry, but that's not how biology or exercising works. You can't just work a specific set of muscles and not the rest of them cause it's like a connected, complex system of pulleys and weights.
Hypothetically? If you hooked some electrodes up to your bum and shocked them the correct voltage and frequency then yes, only your bum would tense and only those muscles would be worked. However, you'd need to take additional calories to build up bum mass or it'd stay teeny tiny. Eating calories makes you get fatter/bulkier all over, so you'd need to do cardio to counterbalance it (and that would make you muscular in the legs/arms cause you're taking on calories and working muscles) or eat exact amounts of calories to a very carefully balanced degree. And even if all that worked? Congrats, you've got a perfect butt that still needs perfect diet and shocking to maintain, and that is probably difficult for your feeble legs to support so they'll start building muscle from the strain (unless you spend most of forever sitting in a chair).
TL;DR – You're a fucking idiot, that's not how real life works. Your options are: do squats and full leg exercises, get saline injections, or be happy with your bum as it is. It is physically impossible to cultivate just one specific part of your body save for injecting nasty shit into yourself or paying a surgeon.