This is true in a superficial sense. But healthy food is generally more expensive than junk food. Gym memberships, exercize equipment, personal trainers, and outdoor recreation costs time and money. It's hard to prioritize self-care when you're struggling with the day-to-day stresses of poverty, like how do I get to work after my car broke down for the third time this month. Poverty is stressful and stress-eating is a thing.

Sure, there is no law of physics that makes poor people overweight, but it is much easier to have a thin waistline when you have a fat bank account. And indeed we observe that in the US, poor people are heavier than wealthier people.