The default tile size for mosaic texture map exports auto adjust so that the width x height of the mosaic matches that of the heightfield.
In other words, if you have 1024x1024 heightfield with 256x256 pixel tile size resulting in 4x4 mosaic and then you generate a texturemap that is 8196x8196 (4x the resolution of the heightfield) then when you go to export the texturemap, the tile size starts off as 256x256 resulting in a 32x32 mosaic. Instead, the tile size for the texture map should default to 2048x2048 which would result in a 4x4 mosaic.