When Advancing a Monsters HD the monster does not gain any size or Atribute modifications. It also gains the +1 stat every 4 levels. I don't believe monsters are suposed to get that bonus stat increase.
I used a Griffon and advanced it to 11HD where it should have gone from large to huge there was no change and it gained +1 stat of my choice.
Windows 7 x64 and Mac OS X Lion
Also I tested this in Version 5.16.4 and it works as expected.
Confirmed - Stefan, you were the last one to play with the relevant code, can you take a look at this please.
Tested Wolf - At 3 HD should have seen something, advanced to 6 HD, the Class wouldn't allow me to exceed 6HD, but Size remained Medium.
Tested against the trunk.
Fixed the size change on levelling up. Note wolf gets large at 4HD in 35e