Why are site images compressed on a 3G network?

Question

mobile connection

I found such a problem in the 3G network of Vodafone (Egypt):
Images when downloaded from the site (in the browser or via curl) are 10 times smaller in size than they are distributed by the server.
I found out that this is happening not only with my site. From all sites without an ssl certificate. pictures come compressed to obscene.
Installed certificates - the problem was resolved. But how are certificates, 3G network and image compression connected?

Answer 1

These are the tricks of the mobile operator itself. It was found in many places, supposedly taking care of customers, saving the traffic they paid for.
The absence of an SSL certificate made it possible for the client to transparently replace the content (in this case, the original image with a compressed one)
Here t-mobile was noticed doing this 7 years ago
Here an operator from the CIS did the same things.

Answer 2

Well, the provider, in order to reduce the load on its mobile network, compresses the images by giving them to the client. And when using ssl, the traffic is encrypted and the server does not know what is there, so nothing can be compressed.