Clothes manufacturers could do consumers a favour be producing their clothes to standardised sizes. If you go into a high street store to buy a medium t-shirt, which fits you perfectly, you would expect a t-shirt of the same size but different brand to also fit exactly the same.
This discrepancy is no more prevalent when you purchase clothes online. It can be very irritating deciding which size would fit you as you obviously can’t try them on.
Also, different countries have their own idea of what mediums should look like. For example, the average person in the US is larger then people form Asia.