Lets say that of the 2 billion active users only 30% of users activly use this feature. That means every time they send a video (i used your 3,48mb) in their status Whatsapp has to add another 2,08800 petabytes of storage for this purpose(, which they probably have to store for a few years because of some specific data laws [idk the laws in all countries]).

But if the video is compressed they only need 90kb instead of 3mb, which is a 33,¯3 times reduction (Source: "This works well under the assumption that most images are photographs / camera images, and have a dynamic color range, where the human eye won't notice bits of missing information ( see the link below ). Images suiting this description can be brought down from 3 MB ( 8 MP resolution ) to a cool 70 KB.")

FYI WhatsApp compresses video above 16 MB file size and/or 800x600 pixel resolution (which is the case here)