Diamonds can be made smaller but not bigger. So the size is a major factor for the price. The price increases exponentially as the weight increases.
But some basics first. The weight of a diamond, often called the size, is measured in carats. One carat is 200 milligrams. No one will tell you to buy a diamond in grams, rather a diamond is sold by carat size.
A 0.50 carat diamonds, certified by the GIA, with color F, clarity VS1, with an excellent cut, will cost around $1400.
But double the size, a 1.00 carat, GIA, F, VS1, with excellent cut, will cost around $6000. Yes, that is 4 times as much for double the size.
Now, a 2.00 carat for same qualities is getting very rare, and will get to over $30.000.
Note, this is the total price