Theres probably no specific reason your gecko is smaller... geckos are individuals like people and grow at different rates I got two babies back in February and both have been slow growers and only now the biggest of the two is around 8 grams and the smaller of the two around 5 grams..... I only have a couple of my rhacs that will actually eat crickets the others ignore them.72-73 degrees is perfectlly fine and well within their temperature range to be comfortable