It's been my experience that while T's in general will lose some weight when they molt, if they were rather plump before they molted, the size difference post molt is mostly negligable. It is when they molt after a long fast time that they generally come out really skinny after a molt.
I guess this would depend on the growth of the spider, because if a spiderling molts the cephalothorax grows substantially, so the bum would appear to be very small (compared to the cephalothorax). If the spider is mature or not really growing the bottom would not appear to be so small.
What about this thought?