The face-up size depends on the shape of the gem, its weight and the proportions of the cut. Face-up area is a measure of the size of the gem when viewed from above (as set in a ring).
It tells you how big the gem is at the girdle plane.
1 Ct Diamond (Pear)
1.2 Ct Diamond (Round)
9.05×5.45 mm →
37.5 mm² ← 6.91×6.91 mm
1ct Diamond Pear (9.05×5.45mm)
1.2ct Diamond Round (6.91×6.91mm)
Difference: 4% (1.49 mm²)
1.2ct Diamond Round (6.91×6.91mm) has approx. 4% more face-up area than 1ct Diamond Pear (9.05×5.45mm).