Is being obese a sin in Islam when you can lose weight but are just lazy?

So the first thing to remember is that our bodies are from Allah subhanahu wa ta'ala. They were given to us and we are responsible for them. Which is why, for example, we're not allowed to have or make permanent changes without valid reason to our bodies. And remember the Prophet Muhammad sallallahu alayhi wa sallam said that our bodies have a right upon us as well. So if we are not fulfilling that right in terms of whether it be sleep or whether it be food or whether it be exercise and so on, and we are doing something which is detrimental to our health, whatever that may be, then yes, potentially we may be accountable for that, for not holding up that right and responsibility of our bodies that were gifted to us by Allah subhanahu wa ta'ala.