Adapted from a question originally answered on Quora.
Personally, we would be thrilled to have some giant-sized bug friends around, but alas, it is not meant to be! There are three main constraints on the maximum size of bugs: oxygen availability, exoskeleton strength, and molting.
Oxygen Availability
If you've ever seen a movie featuring human-sized spiders or giant bird-like dragonflies, you might be thankful that those creatures exist only in the realm of science fiction. But you might be surprised to learn that a long time ago, huge bugs actually existed! To understand why, we first need to understand how bugs breathe. Bugs don't have lungs like humans, which hold air and allow the blood in your veins to pick up oxygen as it circulates through your lungs. Instead, we take in oxygen directly through breathing holes on the sides of our bodies called spiracles. These spiracles are connected to tubes called tracheae that carry the oxygen to our cells. Tracheae are limited in length, and so the larger the bug gets, the less able it is to get oxygen to its cells, and we can't live without oxygen!
Back in the Paleozoic era (245-570 million years ago), insects were larger than they are today because there was more oxygen available in the atmosphere, meaning their tracheae had to do less work to oxygenate their cells. If you had been alive back then, you might have looked out across the lush landscape and seen a dragonfly the size of a hawk, or had to fend off cat-sized cockroaches from stealing food from your pet's dish.
Exoskeleton Strength
Bug exoskeletons (the hard "shell" on the outside of bugs, made of a tough, fibrous substance called chitin) simply aren’t strong enough to support large sizes. Our legs are basically hollow tubes! If we were as big as house cats or humans, it would be a bit like trying to hold up a large water balloon with a paper straw — the straw would bend and collapse under our weight. As a hypothetical large insect grew, its body weight would increase at a much faster rate than its dimensions, rendering its spindly legs incapable of holding it up let alone moving it along!
Molting
Because bugs have hard exoskeletons, we cannot grow by continual small increments the way other animals do. Instead, we must shed our outer shell to expose a soft, expandable shell underneath that's slightly bigger than the old one. This new shell hardens over a few minutes to a few days, but while it's hardening, we are vulnerable to being injured or attacked. Molting is already a pretty perilous and time-consuming process for normal-sized bugs, so imagine how difficult it would be to shed an exoskeleton the size of a cat! It could take us months to wriggle out of our old shell, months during which growth abnormalities would set in or predators could easily snatch us up.
Small But Mighty!
As much as we would love to see giant pet beetles walking down the street, we actually like being small! Being tiny comes with lots of advantages that other animals don't have. It doesn't take much to feed us, so many bug species can eat the same plant their entire life and never run out of food. When a predator is chasing us, we can fit into tiny spaces between rocks, plants, and other structures to escape and hide. Our size also means that we are very efficient flyers! Being so small means that many bugs can live in one place without depleting its resources, whereas larger animals need more territory to roam, eat, and live.
We may be small, but remember: we have a big positive impact on the environment, so we're worth keeping around!