Go to top triangle's top right,
then cast your eye across three squares to the left,
and then down to the triangle.
Do the same for the lower triangle.
See the tiny offset of difference?
There is a different curve going on
between the two triangles' hypotenuses.
The top one with a smaller area has a concave hypotenuse.
The triangle below with a bigger area has a convex hypotenuse.
This accounts for the triangles having different areas.
The area difference would be represented
by the white square in the bottom triangle.