I definltely have 30sec sketches that look like this! Proportion is one of the hardest things to manage in such a short time. I find using a line of action can help mitigate this. It can mark out the space you're going to use on the page, so you don't get to bottom and realize there's no room for legs. It can also help you take in the whole gesture at once, rather than reading the pose from top to bottom, which can also cause proportion problems if you lose track of scale.