I'm about to decide what size of house to build on a 100 tw plot and need some advice on cost.

What I want is 1 story, 2 bead, 2 bath.
What difference in cost would it make if lay it out on 80 and 120 m2 ?

It is common in Thailand to estimate construction costs by m2.
Builders always talk m2 and everyone says that cost strictly depend on the building size. This doesn't make sens to me. If I do 80 m2 for 800k, does this mean the cost would go up to 1.2 mil for a 120 m2?