When you rotate the triangle around it's left edge,
you are tracing out circles from the base to the tip.
The circles at the base are "much larger" than the circles at the top,
in your sketch.
The cone volume is the integral of the areas of the circles.
For convenience, turn the triangle upside-down,
select a smaller radius about halfway up the triangle,
letting "h"=the height of the smaller triangle formed.
Then, in my sketch, the circles are much larger at the top.