Is the property fee legal because the house area is poor?

It is definitely illegal, because the purchase contract has indicated the area of the house you purchased. As for whether it is reasonable or not, it depends on how you understand it. If it is for investment, you can go through legal procedures and ask the developer to refund money or even compensate for liquidated damages. If you live by yourself, it doesn't matter much without it. You can consider negotiating with the developer to solve it. As for the amount of compensation, it depends on how you talk to the developer. Another reminder, the developer had better give you cash and pay the property fee himself. Because developers often say that they will offset the property fees, the result is to put pressure on the property and eventually give them relief. It is the developer who benefits, not you.