Answer
A very shrunken abdomen can indicate dehydration or poor condition; an extremely swollen abdomen increases physical risk. Judge body condition alongside species build, sex, recent feeding and the last moult.
A well-fed terrestrial
For many terrestrial tarantulas, the abdomen should look full and proportionate without becoming a huge, taut weight behind the carapace. Stop feeding for a while if it becomes excessively enlarged.
Mature males
Mature males often have a smaller abdomen and leggier proportions than females. Do not try to feed a mature male into the body shape of an adult female.
After a moult
The abdomen can look noticeably smaller just after moulting. Keep water available and wait for the fangs to harden before feeding.
When a small abdomen is concerning
Review hydration, feeding history and behaviour if the abdomen continues shrinking, especially with poor coordination, weakness or an abnormal posture.
Never handle just to inspect
Use normal viewing or a photograph. Handling a heavy or weak tarantula adds fall risk and rarely gives better information.