chalky gobies

[smoore-gt225]

hi i have 2 chalky gobies i dont know the sexes i was told you cant tell, 1 is twice the size of the other though, how can you
tell if they might be breeding as the larger 1 is in his hole in the sand under a rock been in there about 2 days and he looks like he is guarding something and the smaller 1 is sometimes lying across the entrance. does anyone know what might be going on
heres hoping they might be getting it on
cheers steve