I don’t think leftists think weight is exactly equivalent to class, race, gender or sexuality in terms of systemic oppression. That doesn’t mean that people don’t experience weight discrimination or fatphobia.

In the west, the working class is less likely to be able to afford nutritious food than the upper class. Food deserts are most common in impoverished communities and this is absolutely by design. It’s also worth mentioning that people can be both fat and malnourished. Malnutrition isn’t just caused by the quantity of food people eat, it can also be caused by a lack of quality. In the US in particular, many fat people are both poor and malnourished; there really isn’t a contradiction here.

Cooking your own meals and building an effective exercise routine can be time consuming. People who need to sell their labour to survive generally have less free time to spend on these kinds of tasks. Fatphobia is an issue that is more likely to impact the working class and marginalized communities, at least in the west.

There’s also a LOT of intersection between racial discrimination, fetishization and body shaming. The horrific objectification and exploitation of Saartjie Baartman is an (extreme) example of this but it isn’t difficult to find similar instances of this kind of hypersexualization and abuse directed towards minority women who don’t fit into a specific eurocentric idea of health or beauty. This also overlaps pretty heavily with medical discrimination, which Othello already mentioned.