Looking at a file in the explorer, the size column shows 5,138,005 KB.
Opening the file properties shows 5,261,316,057 B (or 5,261,316.057 KB).
What size is the column showing? Where did it come from?
In Explorer on Windows 10, the "Size" column shows file size in KiB (kibibytes). One kibibyte is 1024 bytes. On the other hand, a "true" kilobyte is 1000 bytes. That is where the discrepancy is coming from.
To show this is true, multiply 5,138,005 KiB by 1024 and you should get the number of bytes.
5,138,005 KiB * (1024 B / 1 KiB) = 5,261,317,120 B
Additionally, the "Properties" dialog shows you the "Size" and the "Size on disk" which are two separate things. Due to the way computers store information, the actual size of a file on disk can be larger than the actual information itself.
You'll notice that number calculated above is much closer to the other that you gave in your question, but not identical. I suspect this is because you got the "Size" from the properties box instead of the "Size on disk".