Caterpillar variations

The two pebble prominent larvae I brought indoors from the window box sallows the other day are now nearly fully grown and resemble one another rather little.  One is much larger and I would estimate about two and a half times the  size of the other.

It could be that the smaller, paler one (below) will be a male, but I am not sure at which stage sexual differentiation in moths and other invertebrates occurs.  The size of the lower one, by the way, is overestimated by these photos and remember, the head is on the right