I think it is genuinely the software. I don't think the instruction set matters all that much. Everything was much more bare-bones 20 years ago, from terminal emulators to browsers to desktop environments to word processors to code editors to games to media players. I wouldn't call the change bloat exactly, but software projects have grown immensely more robust. The kernel is constantly gaining new device drivers and rarely shedding them. The browser has evolved into an operating system unto itself. Instead of just building X11 and a lightweight window manager like XFCE we now have wayland compositors - which, while much more architecturally simple, carry the baggage of XWayland for compatibility anyway. We have a whole slew of graphics stacks from Vulkan to OpenGL to GLES, a whole slew of GUI toolkits from Xlib to GTK+ to Qt to wxWidgets to FLTK (each with dozens of language bindings), a whole slew of new programming languages such as Go and Rust along with their own whole ecosystems of libraries and dependencies, a whole slew of additional daemons running in the background to make basic shit like plug-and-play device detection, power management, bluetooth, etc. work. We've got more filesystems, more audio/video codecs, more compression algorithms, more file formats in general. Syntax highlighting which used to be done by naive Scintilla controls is now managed by robust language servers. The coverage provided by compatibility layers like Wine has only expanded, and targets more operating system versions than even existed when it was introduced.

Don't get me wrong, a lot of it is bloat too, but the state of the art has shifted profoundly since the dawn of the millennium.