Hams do not trade as one product. The USDA pork report breaks trimmed selected hams into weight brackets, and the two that carry the reported trade are the 20-23 pound and 23-27 pound classes. The weight is the green ham itself, before curing, and it is set by the hog: heavier carcasses hang heavier hams, so the mix available on any given day is a byproduct of slaughter weights, not of anyone's ordering decisions.
The workhorse and the scarcer spec
The 23-27 pound class is the market. Over the last five years it has printed a median daily reported volume roughly fourteen times the 20-23 class, which makes it the default input for large curing and further-processing programs and the bracket that effectively sets the ham market's direction. When traders talk about hams without qualification, this is usually the number they mean.
The 20-23 pound class trades thinner and, most of the time, slightly higher: it has held a small premium on about seven in ten trading days across the last five years. The premium is a scarcity and fit story. Lighter green hams are the minority of a kill that has trended heavier for decades, and a program built around a lighter finished piece cannot substitute a heavier ham without changing what comes out the other end.
Reading the spread
Because the supply of each bracket is pinned to the slaughter mix, the spread between the classes moves mostly on demand. The useful questions when it stretches: is the light class being bid because the programs that need it are competing for a small pool, or is the heavy class being sold down because the commodity curing pipeline is full? Volume answers it. A light-class rally on its own thin volume is a niche squeeze; a heavy-class break on big volume is the ham market itself softening.
Seasonally, the whole ham complex has its own well-known rhythm around the holiday curing calendar, which is a separate read covered in the ham holiday premium article. The size-class spread is the within-market view: same product family, two brackets, one spread that says who is bidding.