The mint made 72 million small head 2004 twenty cent pieces in 2004, but just 400,000 examples of the large head version. For this reason, many collectors conclude that the small head is far more common than the large head type. What many collectors don’t realise, however, is that all the 2004 mint sets contain the large head type, meaning that there are almost 68,000 high-quality UNC examples out there for collectors to slab. Meanwhile, the small head variety was released into circulation, so finding high-quality UNC examples is tough.
The 2004 small head twenty cent: Scarce in high grade, because none were preserved in mint sets.
The 2004 large head twenty cent: Despite a mintage of 400,00 pieces, this variety is common in high grade because all 67,795 mint sets have one.
If you want to see some hard numbers: Refer to the PCGS Population Report here. The highest graded large head (the supposedly rarer coin) is MS70. And there are dozens in grades MS67 up. Meanwhile, the highest grade small head is MS66, with the bulk mostly in MS65.
It is little things like this that can help the collector look passed mintages, and unearth true rarities (and perhaps jump onto a bandwagon before everyone else does).