AM437x ZDN (N491 BGA) Footprint Dimensions
I have received an answer on this from TI forum. I am posting it here:

The AM437x nominal ball size is 0.42mm +/-0.06mm. The IPC recommended pad size reduction for this ball size is 20%, which would give a pad size of 0.336mm. However variation of +/-0.05mm is allowed for this pad size, which makes 0.28mm pad size absolutely acceptable.