Hi! Sewist for 30 years, fat person for a lot of those years.
You are not one size everywhere so you shouldn't peg your self esteem to a number. Sizing isn't standardised anywhere and it'd be impossible to do it anyway.
Every manufacturer starts with a "block" that's usually about size 8 for straight sizes then they grade the pattern up and down from there (enlarge or make smaller). Some manufacturers grade up to size 22 from their original block. This is why large sizes can have wickety wickety wack dimensions. Humans put weight on in uniquely human ways and embiggening some shapes on paper will not suffice.
So much of clothing manufacturing is engineered with a wide demographic in mind, and you should think about it as each size being an average of a population's measurements and spread out over a range of sizes.
Many brands use multiple factories to create different garments, and this is why your size can vary within one brand, too! Also hot tip - this happens because the buyers pressure the factory owners and managers into lowering prices as much as possible, playing factories off against each other. This leads to the human beings who make your clothes earning a pittance and working nearly 24 hours a day with one day off a month. Every piece of clothing is hand made. Remember that, and remember who is really paying the price when you buy a tshirt for $5.
Sizing does not reflect you as a human, your worth, your attractiveness, how nice you are, your health etc. It's literally statistics. Marketing and capitalism have twisted it into something attached to our self worth and ego so we buy more clothes in an endless pursuit of trying to feel better about ourselves.
Separating yourself from this system is powerful for your self image and your bank account 
PS you don't need as many clothes as you think you do.
PPS learn to mend at the very minimum.