Too much difference in tire diameter may put stress on the AWD components and cause additional wear. How much difference in tire size is acceptable is a topic of considerable debate. The BMW x-Drive system is reported to tolerate greater differences in tire size than some AWD systems. Some suggest a difference up to 1% in tires is acceptable for the BMW x-Drive. See links below for some related discussions.
https://www.tirerack.com/tires/tiret....jsp?techid=18
http://x3.xbimmers.com/forums/showth...=replace+tires
http://x3.xbimmers.com/forums/showth...=replace+tires
You did not state your tire size, so I am unable to make the calculation of the exact differences in your tire sizes.