Maybe that person is comparing it to more traditional chat clients like Pidgin or Hexchat. Right now on my Manjaro desktop, Telegram is using 230mb of RAM, while Pidgin is taking 46mb. That being said, its a hell of a lot better than Slack, which could use up several gigabytes if you were in multiple groups.

One should also check Telegram settings for media downloads\autoplay. This may change memory consumption drastically.

on my desktop tg would routinely eat up to much memory so that i am now starting it with a script that sets a hard memory limit. haven't had a problem since.