When you work your lower body rep per rep, you burn a lot more calories churning out squats than biceps curls.
And while it’s good to work on a sculpted upper body, your largest muscles are in your legs. Plus, by working bigger muscles, and thus, more muscle fibers, you create more of those micro-tears your body has to spend calories to repair.
Furthermore, when you do work your arms, stand up whenever possible. As soon as you sit down, those big calorie-burning muscles will become dormant.