also in the wild - T's have more space to move so get more exercise (if T's do) whereas in tanks are limited

Hogwash. Most spiders (females) will spend their entire life (in some cases 20 or more years) in a burrow and NEVER venture far away unless the burrow is damaged in some way. Exersise has nothing to do with the "weight" of a spider.

There is no such thing as a problem T, only problem keepers. Likewise for children, only problem parents. Tarantulas will not "loose weight" because they are active, they will starve due to lack of food or dehydrate due to lack of fluids. They get a lot of their flueds from food, but drink and/or absorb through their booklungs also. The oposite is also true. Because Tarantulas are mostly inactive, some keepers find the activity assosiated with feeding stimulating, so they keep feeding the T to see something happen. I am yet to see a spider open it's enclosure to go get itself a cricket or roach. If you feed spiderlings once a week and keep them hydrated, they will be fine. If you feed bigger spiders once every two weeks, also good. If you pace yourself, your spiders will never look over ripe grapes.

My 2c.

I agree scarab.
I definitaly over-fed some of the my Ts and I don't like how they looked. I saw photos on the net on over-weight tarantulas dragging their abdomens so much that they lose all the hair underneath.

I am being much more strict now with regards to feeding, but even more dilligent on hydration.

I have become quite strict too. My T's are fed so their opisthosoma's are chubby, but in proportion, not over swollen and looking like they are going to burst, as slings tend to look when over fed.

I feed my Slings every 2-3 days, usually 1-2 crikets depending on the size n'all. And some days they just skip on hunting so i remove the happy crickets. They are noway as err chubby as the pics above. I have read numerious post on it being impossible to overfeed slings. On another note, feasting encourages fast growth but shortens the lifespans of most T's. So i stick to the hitler diet schedule ;]