The OVA format is basically a tar file that contains:
ovf,manifest,certificate and required vmdk files. There is some optimizations done (I am personally not sure of the details) to conform with the requirements of the OVF. The specific optimization in the OVA case is referred to as streamoptimized format and is usually more efficient: https://www.vmware.com/support/developer/vddk/vmdk_50_technote.pdf
The ZIP format is not doing any form of processing and just zipping up the sparse files.