I agree with you, and I know it IS possible for them to expand existing worlds. Here's the proof: https://forums.thesims.com/en_US/discussion/comment/17503943#Comment_17503943
I guess we only need to make too much noise for them to hear us. Anyway, they say that the worlds are smaller cuz it would lag our games, so why don't they give us an add/remove feature so that we can manage how many worlds we have in each save, which worlds we have in each save, have multiple copies of the same worlds, etc... Maybe they could even add a way to share our modified world to the gallery (kinda like a custom save file). It would work exactly like the Recommended Sim Count, that warns us when we have TOO many sims and the issues it could cause.
My laptop is low end as well, and I've never had any problems with worlds because of their size. My only problem was in Brindleton Bay, in the biggest lot, and probably cuz they did something worng in the coding, cuz all the other big lots run perfectly smooth to me.