Can't delete sprites from atlas

[justinISO]

I have an atlas that was made using NGUI Atlas Maker that I needed to update some sprites on. When I tried to update it I got

Operation canceled
The selected sprites can't fit into the atlas.
Keep large sprites outside the atlas (use UITexture), and/or use multiple atlases instead

So I tried deleting some sprites from the atlas and I get the same message. How was I able to make that atlas normally and now it says there is no room to even delete sprites?

[ArenMook]

You probably changed your platform from PC to mobile, so your atlas effectively went from 4k to 2k.