See, suspension of disbeleif failed for me on that small artifice, because you can't have a giant spider happily clinging to a roof in one scene and then suffocating from its weight in the next - if square-cube law and the spiders weight is an issue, then make sure that gravity applies to spiders on the ceiling too!! [
Science interjection - you've got it a little messed up.
Arthropoods have two separate issues as they get large.
One is weight - the materials used in exoskeletons are not particularly strong. It cannot carry the same weight that bone can - so, if they get too big, arthropod exoskeletons will break legs, and the like. This can be gotten around by fundamentally changing the materials used in the skeleton, but then you're no longer really an arthropod. You're something else. But regardless, you can't just make a garden spider grow the size of a horse or larger and expect it to bear up its own weight. It will crush itself.
The other is oxygen - their oxygen problem is related to size, but *NOT* to weight. Arthropods do not have lungs and a closed circulatory system to move oxygen through the body. They have an open circulatory system, in which oxygen isn't so much pumped through the body, as it more diffuses through, with only a little mechanical help as the animal's muscles move. If the body is too large, you can't diffuse sufficient oxygen into the interior tissues from the surface of the body in this manner, and tissue deep in the body would die. This is what seems to have been happening to the ballroom spider.
Interesting point - this mechanism is in part dependent on the percentage of oxygen in the atmosphere (actually the partial pressure of oxygen, but close enough for this discussion). So, raise the percentage of O2, and the bugs can be bigger. Back tens of millions of years ago, the percentage of 02 in the atmosphere was higher than it is now. And bugs got bigger. MUCH bigger. Still within the limits of the exoskeletal materials, still way bigger than the centipedes you see around today. Bigger than coconut crabs, which are the largest land arthropods these days, who can weigh as much as a house cat and whose legs can span three feet.