Probably extreme compression, their 4K streams are very low bandwidth.
So, about 10GB or less on Netflix to 30GB or more on AppleTV+, dissected by DPI on my TP-Link Omada Gateway.
And indeed, i think it shows - i can't notice any banding or moire effect on pretty much any AppleTV+ content, while it is as clear as night and day that Netflix compresses the hell out of their content.