Actually I am not sure about this. Car maybe, but Computer?
The most material on your dektop is cheap aluminium and similar.
The thing that makes computers so environmentally damaging is the gold and even more the very scarce other stuff. You also have that in your phone, maybe less, but 5 times less? I doubt.
Of course that is purely academical because there are very few computers that survive 15 years ;)
Only the hand full anecotical Linux mail server that stop working after years and years and nobody can find it because the building it is supposed to be standing in was demolished 3 years ago.
It certainly depends on the specifics of the computer, but I'd wager that a computer does have around 5x more waste products than a cell phone. Cell phones are mostly battery and glass/aluminum. Now if we're comparing a super high end phablet phone to a really compact chromebook, you're probably right and there's not a huge difference. High end phablet to high end desktop (or even laptop), I suspect the ratio is near 5. Don't forget about the monitor and cables to go along with that desktop.
And yes it's academic, my point was that the typical person probably creates considerably more waste with computers or cars than with cell phones, even though they're replaced more often.
Meh. I've got several 10+ year old computer monitors that still work great and see daily use. I've got some 20+ year old keyboards/mice doing the same. Desktop computers are designed to be upgraded when they start showing age . . . even someone with no idea what they're doing can easily add RAM, swap out a processor, add another disk drive, change graphics card, etc. Even if you have to replace the whole board, CPU, and RAM . . . you still end up with a reusable case, a reusable PSU, reusable fans, reusable optical disks, reusable HDDs. Power cables are standardized, SATA cables are standardized, IDE cables are standardized, USB cables are standardized, VGA and DVI cables are standardized. None of them need to be tossed if a computer goes bad.
Phones are designed to be disposable in their entirety every couple years.