Hi,
It is because the latest data is always added to global map, thus making it resize if the robot is near the boundaries of the map. You can try with this to see the difference (this not add the latest data when the robot is not moving):
<param name="map_negative_poses_ignored" type="bool" value="true"/>
Another way to avoid resize is to provide a minimum size of the map:
<param name="grid_size" type="double" value="50"/> <!-- 50x50 meters initial map -->
Note that resize message is not bad, it is just for info.
cheers