The problem could be down to extents of your input data. You say they are similar extents that sounds a bit subjective to me. If you look at the point to raster tool help file you will see that it honours the extent environment setting. Try setting that to a common extent that you want for all your rasters.
Another thing to check is: do all you input point layers have sensible coordinates? If the data is coming out of a text file or evil Excel then null rows may get given a 0,0. A single point at that location cannot be seen say at the extent of your data so something has to fill in all that space and it will be nodata values. This would increase the size of the data.
One final check is you say the output is float, well you can be 32bit or 64bit.