property tax is not totally based on the size of your house. it is based on annual value. To quote IRAS,
"The AV of buildings is the estimated gross annual rent of the property if it were to be rented out, excluding furniture, furnishings and maintenance fees. It is determined based on estimated market rentals of similar or comparable properties and not on the actual rental income received."
To put it simply, a 50 years old run down 6000sqf house will have a much lower AV than a brand new 6000sqf house since the brand new house will definitely fetch more rent compared to the old house.