PDiffs
Try to download deltas called PDiffs for indexes (like Packages files) instead of downloading whole ones. True by default. Preferably, this can be set for specific sources.list(5) entries or index files by using the PDiffs option there.
Two sub-options to limit the use of PDiffs are also available: FileLimit can be used to specify a maximum number of PDiff files should be downloaded to update a file. SizeLimit on the other hand is the maximum percentage of the size of all patches compared to the size of the targeted file. If one of these limits is exceeded the complete file is downloaded instead of the patches.