This expression is from the movie There's Something About Mary:
She put a lot of weight, about a deuce and a half.
Is this used to refer to overweight people? The only reference I could find is from some American Army truck.
I hadn't come across this expression before, but I have to say it doesn't seem very apposite to me. My car weighs nearly 2 tons, so a 2.5-ton truck doesn't sound particularly big to me. Wikipedia agrees, saying there are 8 truck categories — the lightest of which is for anything under 3 tons.
Anyway, it's not exactly "common", but here are a few dozen instances of "she's a deuce and a half" showing that it's far from unknown (even if the scale factor is a bit off).
Per comments below — unquestionably "a deuce and a half" is military slang, but it refers to the load-carrying capacity of the truck, not total weight. And Urban Dictionary's 2.5 x 100lbs = 250lbs is, well, an urban legend sort of rationalisation.
In this context "deuce and a half" has nothing to do with the military truck. It is instead slang for a large person in the neighborhood of 250 pounds. Urban Dictionary defines the term as "a woman that weighs in the neighborhood of 250 pounds." Men also can be described as such, including news about the football player for which "recent internet rumors had... pushing the deuce-and-a-half mark (a.k.a. 250 pounds)." Yahoo answers also features a reply about "cool army nicknames" in which a fellow notes that his was deuce and a half because "I'm a big guy over 6 feet and while I was in the army I was about 250 pounds ex-football type."
On a related note, "deuce and a quarter" is the nickname for the Buick Electra 225 automobile, which was produced from the 1950s-80s.
The term deuce and a half is for certain, slang for the military truck. And if you do a Google search, you will see it is not some light weight truck – it's BIG.
So, when used in the reference of a person's weight you can be sure it's not a term of endearment. As posted above, it could represent 250 pounds (200 for deuce and 50 for the half) – still it is not a term of endearment.
(Just a warning: if you say this to a woman, duck, because you are about to be punched).