> 
> 
> It's close but not entirely correct. The computation needs to include the extra header word for "large" instances (> 255 bytes). Squeak has three possible header sizes:
> 
> 	1 header word  - used by compact classes
> 	2 header words - used by regular classes
> 	3 header words - used by "large" instances

mariano you should have a look at the object engine chapter written by tim (available on my web page)
it explains the different headers.