Why is my minecraft world file size getting smaller as I build it?

I’ve just converted my 'xbox 360 version' world into a 'Xbox one (bedrock)' version. in the conversion, it seemed to lose 4Mbytes, based on the filesize from my Xbox one.

Why is this happening, and am I losing data from my world?

I believe there are two reasons for this.

Firstly, the two systems (Xbox360 {FAT32} and Xbox One {NTFS}) use differing Disk Formats by default. These disk formats will have different setups, which mean that the stored data apparently occupies a different number of 'blocks' on the drive. Moving the data from one to the other could account for SOME of the apparent reduction in data.

Secondly, the two game versions likely use different region file formats (world saves). I can't find data on specifically what they are called, but as the two systems differ, it would make sense the amount of data stored is also different.