The cluster size, the smallest amount of disk space that can be used to hold a file , would be the same for NTFS as FAT32. This is a confusing point, and this makes this a good question. Why then would there be differences in formatting sizes? As already mentioned, there are more features built into NTFS than FAT32. NTFS provides things beyond FAT such as metadata on files (file name, creation date, access permissions and sometimes contents), security access control lists, and file system journaling. These features improve reliability, security and flexibility; but they also lead to slightly higher space constraints, which one sees when the cluster sizes are the same. It is important to note that if your USB were larger, NTFS tiers for cluster sizes increase more slowly than FAT32. So, you would see the reverse of your observation in larger USB drives .
The largest contributor to the discrepancy in size difference is the difference in sizes of the tables that are used to keep track of which files are where on the USB drive.
FAT32 uses the File Allocation Table, which is simpler and smaller than the Master File Table in NTFS.
"NTFS reserves 12.5 percent of the volume for exclusive use of the MFT until and unless the remainder of the volume is completely used up" .
The complexity of comparing the reserved space is beyond the scope of this answer, but I can refer you to further search parameters like "reserved space," "MFT versus FAT size, and "boot sector."