Yes, the maximum attachment size depends on the user group.
You can change the attachment size for a certain user group by following the steps below:
Admin CP >
'Users & Groups' Tab >
Groups > Click the name of the usergroup that you want to increase the attachment size of >
'Forums and Posts' tab > Scroll down to
"Attachment Options" > In the box under
"Attachment Quota:" insert the size you want the usergroup to be limited to uploading in KB or set it to 0 for no limit.
Finally click
'Save User Group'.
Sidenote: You may want to post any future MyBB questions over at the official MyBB Forums, http://community.mybboard.net/ - we can assist you better there.
Regards,
Chris.