Sam is a shape-shifter. All his appearances as a non-human have been in the form of a dog. The dog, clearly seen in many episodes, weighs, at most 50 pounds. Sam would appear to weigh 160-170 pounds. While the laws of physics (conservation of mass) would beg the question of where the rest of Sam's mass goes, this is magic. Normal laws of physics don't apply.
Tara protests the price she's quoted is more than double what her mother paid for the exorcism. Tara's mother paid $445 and Tara is quoted $799.99. Her math is wrong.