Drag and Drop Feature & Limitation

Dear All,

We tried to migrate documents from Shared File Server to SharePoint using drag and drop feature to document library. 

Unfortunately the documents are not being copied in the subfolder & sub-subfolders.

We hold the 1st level of folder (including subfolders in this folder) and drag to document library. 

Does SharePoint think that it's considered as a single file? Hence the limitation will be in 10 GB?

Please advice and help.

Thanks.