If the abdomen was shrinking then there was a problem with your spider.

When last did your spider molt?

Can you take a picture of the abdomen, your spider might be in pre-molt!

Only leave the pre-killed prey item in for a day and then remove.

HaHa! Klippie no dont' try that, just take my remark as something born of sarcasm / frustration...

Still hasn't eaten anything.. and definitely isn't in pre-molt... sibling is now easily over 250% bigger..

Kills its prey item and leaves the carcass uzi

How long do you leave the killed prey item in the container?

Take a picture of the spiders abdomen, should be able to see if it is in pre-molt.

Thanks Cascade, your input is much appreciated bud, in fact you inspired me to grab my cam.. the pictures will speak for themselves but this is the run down.. spider in question is now in 4th instar, got them both at 2nd. Sibling is at 5th. Skinny hasn't eaten since midway through 2nd instar. Has eaten once since I got it. Sibling is having no problem with food. Spider in question seems timed and moves away from prey, sometimes raises / waves front legs in threat. Find it killed a pin head the other day but didn't consume it, simply left it. Pictured are the spider in question, 10c piece and its sibling. Got a close up of the opisthosoma for you.

Fire away Sir.