ESXi doesn't support Workstation/Player sparse .vmdk file formats, but uses a two file format. One of the .vmdk files (<vmname>.vmdk) is the descriptor file and the second one (<vmname>-flat.vmdk) is the data file. If you want to upload a .vmdk file to an ESXi host manually - i.e. without using e.g. VMware Converter - you need to use the vmware-vdiskmanager command line tool to convert the virtual disk into the ESXi compatible format (4 - preallocated ESX-type virtual disk) prior to uploading.
André