Yes, it is a matter of ROM size.
The HP-35 has 7680 bits of ROM, and the HP-45 has 14336 (not counting an additional 2560 occupied by the undocumented timer mode).
The HP-21 has 10240 bits of ROM. That combined with the increased efficiency of the second-generation "Woodstock" processor architecture allows it to have more functionality than the HP-35, but not as much as the HP-45.
The HP-25 and HP-25C have 20480 bits of ROM, the same amount as the HP-45 (including the latter's timer mode), yet it packs in a lot more functionality, including programmability, thanks to the efficiency of Woodstock.
On the other hand, the HP-29C, which superficially seems like it has only relatively minor feature enhancements over the HP-25/25C, uses twice as much ROM, at 40960 bits.
Edited: 24 Mar 2012, 7:20 p.m.