Everyone would like more legroom. No one wants to pay the extra money. Most (if not all) UK charter airlines now offer some sort of premium class, for which you get a much bigger seat as well as some other frills. It does cost money though. On short haul around £50-60 return. On long haul quite a bit more. Some pay, some just buy the cheap seats then whinge.
If you are large then you have a choice. You buy yourself two seats or you pay extra for a seat with more room.
If you find economy uncomfortable then buy a Business Class seat. If you can't afford a business class seat and find you can't fit in an economy seat then don't fly. No-one is forced to go on an aircraft.
Economy class is a chance for those who don't mind putting up with a bit of discomfort the opportunity to go to places they otherwise could not afford to visit.
Just look back in time. Flying used to be a luxury, with individual service and a "luxury" cabin. And few could afford it. So economy class and the charter airlines came in.
Perhaps airlines should advertise that a standard seat is available at "x" price, but if you want a smaller seat its half the price. Guess which cabin will be full?