Size on disk error

[stevecrt]

LS420D (Seagate HD's) FW Version 1.31-0.92 Win 8.1

Hi

I have a just copied some folders to my new LS420.
One of them is 1.2 TB in 'size' but 14.35 TB according to the 'size on disk' under Win 8.1 properties option.
Despite the fact that the drive is only 4 TB I find this confusing.
Should I be concerned that at this early stage the LS is faulty?
The LS still has space left, and I can still copy other folders to it.
Notably, Linux reads it as 1.2 TB but no size on disk is available and the original source, my older LS Pro duo, does not give this 'size on disk' anomaly.

Thanks in advance - Steve