The maximum size of the internal code for an individual procedure in Visual Basic version 3.0 is 64K bytes. Microsoft Visual Basic version 4.0 for Windows uses Microsoft Visual Basic for Applications, which generates more generic internal code than does Microsoft Visual Basic version 3.0. This generic code allows for compatibility with a wider range of processors. This makes the internal code larger, so it can cause very large procedures to exceed the 64K limit and fail to compile.
What other processors they may be referring to, I have no idea. But whatever this "generic internal code" does, it can contaminate things. I changed a couple of things in a 2k form using 2003 and it became unuseable (within 2k) due to various errors. Thus, if anyone out there is going to migrate from 2k to 2003, get the KY ready and start limbering up.