Yeah, they actually waste more in this case. The base cost is that of processing of content, which is linear with size (on a category level; parsing JS may have a different constant factor than displaying an image). But in the typical makeup of a website, just how much stuff can be in the 10KB case? Content + some image + a bit of CSS and maybe a JS tracker. In the 10MB case, you have tons of JS code, a lot of which will keep running in the background. This incurs continuous CPU cost.
> and people don't just flip the power switch as soon as a page load finishes
CPUs have power-saving modes, power supplies can vary their power draw too.
Or, for those with whom such abstract terms as "wastefulness" don't resonate, let me put it in other words: if you ever wondered why your laptop and your smartphone drains its battery so quickly, this is why. All the accumulated software bloat, both web and desktop, is why.
But it has. Data transfer and processing isn't free. It works on electricity. You may think that a difference between 10KB (efficient) and 10MB (current web) is meaningless because resources are abundant, and it let you save couple hours of dev time[0] - but consider that this difference is per user, and you saved a couple hours for yourself by making thousands[1] of people waste three orders of magnitude more electricity that they would if you were a bit more caring.
Like plastic trash and inefficient cars, this stuff adds up.
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[0] - Such savings on larger pages take obviously much more work, but then this time gets amortized over all the use the website has - so the argument still holds.
[1] - Millions, for large sites.