The number of bytes added in several diffs are wrong. The bug applies to page histories and user contributions, but does not seem to have any effect on Recent changes.
Here is one example:
MerlIWBot edits Template:Link GA on wikidata-test-client by adding one interlanguage link. In his contributions and in the page history the edit is stated as being 1370 bytes. Those 1370 bytes are actually the size of the template itself, but not the size of the edit. The RecentChanges state that the edit is 30 bytes, which is much more believable.
18:51, 12 May 2012 (diff | hist) . . (+1,370) . . Nm Template:Link GA (Robot: Adding el:Πρότυπο:Link GA) (top)
From index.php?title=Template:Link GA&action=history
(cur | prev) 18:51, 12 May 2012 MerlIwBot (Talk) m . . (1,370 bytes) (+1,370) . . (Robot: Adding el:Πρότυπο:Link GA) (undo)
12 May 2012
(diff | hist) . . m Template:Link GA; 18:51 . . (+30) . . MerlIwBot (Talk) (Robot: Adding el:Πρότυπο:Link GA)
I can name several other examples aswell, but I am going to stick to this one for now.