The expression "fits like a glove" is an odd one, because there are many different types of gloves and only a few of them are going to fit the situation you are in. If you need to keep your hands warm in a cold environment, then you'll need a fitted pair of insulated gloves, and a glove made to fit in the bureau of a dollhouse will be of no help whatsoever. If you need to sneak into a restaurant in the middle of the night and steal a pair of chopsticks without being discovered, then you'll need a sheer pair of gloves that leave no marks, and a glove decorated with loud bells simply will not do. And if you need to pass unnoticed in a shrubbery-covered landscape, then you'll need a very, very large glove made of green and leafy fabric, and an elegant pair of silk gloves will be entirely useless.
Nevertheless, the expression "fits like a glove" simply means that something is very suitable, the way a custard is suitable for dessert, or a pair of chopsticks is a suitable tool to remove papers from an open briefcase, and when the Baudelaire orphans put on the uniforms of the Queequeg they found that they fitted the children like a glove, despite the fact that they did not actually fit that well. Violet was so pleased that the uniforms had several loops around the waist, just perfect for holding tools, that she didn't care that her sleeves bagged at the elbows. Klaus was happy that there was a waterproof pocket for his commonplace book, and didn't care that his boots were a bit too tight. And sunny was reassured that the shiny material was sturdy enough to resist cooking spills as well as water, and didn't mind rolling up the legs of the suit almost all of the way so she could walk. But it was more than individual features of the uniforms that felt fitting-- it was the place and the people they represented. For a long time the Baudelaires had tossed from person to person and from place to place without ever really being appreciated or fitting in. But as they zipped up their uniforms and smoothed out the portraits of Herman Melville, the children felt as if the Frisbee of their lives just might be repaired. In wearing the uniform of the Queequeg, the sibling felt a part of something-- not family, exactly, but a gathering of people who had all volunteered for the same mission. To think that their skills in inventing, research, and cooking would be a appreciated was something they had not thought in a long time, and as they stood in the supply room and regarded one another, this feeling fit them like a glove.