While performing patch analysis for some one-off patches I noticed there was quite a size difference between codelines. (ie. R12.AP.A vs R12.AP.B) For example, take a look at payables patch 8733916. The version compatible with AP.A is 35.4MB while the AP.B version is only 9MB.
A coworker forwarded me a note from Metalink which describes the issue:
Release 12: Why are One-Off Patches so Large? [ID 841218.1]
The note has an interesting chart which shows how many files are applied from a patch based on the codelevel. Basically a one-off patch contains all the files necessary to fix the problem on a base R12 release. So to summarize the note, the more upto date you keep your EBS environment, the less work adpatch has to do in order to apply patches.
Background Info: Adpatch performs a number of tasks when applying a patch. One of them is to compare versions of files supplied by the patch to those in your EBS environment. Only newer files will be copied.
In my case, with patch 8733916, its basically the same situation. R12.0 was released I believe back in Jan 2007, while R12.1 was released in May 2009. So if a bug affects both versions, its not surprising that the patch would be smaller for R12.1 since its more upto date. Now, I would assume this is highly dependant on the bug but I haven’t seen a patch for R12.1 that is larger than R12.0.