Sorry for the inconvenience. The reason caused this issue is that the openssh-sftp client will send the rekey request when so many bytes of data have passed over the connection. And WingFTP's SSH-2 implementation currently does not support session rekeying because we thought the rekey serves no useful purpose, and causes all sorts of interop problems. Anyway, we decide to re-write the whole sftp module in order to has a perfect compatibility. This project maybe take a few months because many source will be added and modified. Hope you can understand.
Besides, if you need to use sftp to transfer the big file, we suggest you use the putty client. It is also cross-platform. And all most is it supports transfer resume while openssh-sftp not supports, this feature is most importantly for transfer the big file.