Content is also getting heavier, but both things aren't mutually exclusive. It's more objective to compare modern software, instead of older and newer ones. Before reddit created obstacles for third-party apps, they were famous for being much lighter than the official one, while doing the same (some even had more features). Now, if we compare lemmy to reddit, it's also much lighter, while providing a very similar experience. Telegram has a desktop app that does everything the web version does, and more, while lighter on resources. Most linux distros will work fine with far less hardware resources than windows. If you install lineageos on an older phone, it will perform better than the stock rom, even while using a newer aosp version. If you play a video on youtube, and the same one on vlc, vlc will do the same with less resources. If you use most sites with and without content blockers, the second one will be lighter, while not losing anything important.
I could go on and on, but that's enough examples. There is a bloat component to software getting heavier, and not everything can be explained by heavier content and more features.