While I've looked into a similar post I made here years back - I have the same problem of illustrator. Seems anytime you go to add a rounded rectangle either with their selector or to modify a square edge rectangle it committs this acts -- in effect transforming a 38px by 40px image into 113px by 115px all on its own when saving for web.
If this is a "bounding box" that's been added I don't know, but it seems to be invisible.
Tried selecting it, going to object and then ungroup, which only worked a single try. Apparently there's no clipping mask to release.
How does everyone across the planet using the thing tell Illustrator to bring it back to the actual, intended size? Am saving as .ai and then .gif for the web.