This is effectively not all that different from a sugar tax.
Excuse me, what?
It's absolutely not similar to a sugar tax.
You can make the conscious decision to eat sugary foods, or avoid them. Taxing sugar is like taxing tobacco or alcohol, it's essentially a tax on a legal drug. You can live your entire life without consuming any added sucrose.
Increasing the price for larger clothing sizes does not help overweight people at all to be dissuaded from being overweight or eating less sugar - not only do you need clothing for, you know, actually
living, working and not getting arrested, having less money to spend on food means there's less money available for healthier diets; after all shitty food is much cheaper.
What you're basically doing here is saying that overweight people - including overweight children that may not even be responsible for what they are being fed - should be
punished for being overweight by charging them more for essential items of living, rather than getting assistance in losing weight.
Your suggestion would be akin to charging fat people more rent because they take up more space in an apartment.