It's not something to buy or not, it's just how it is; the space inside a glove is going to be strongly connected to the weight of the glove.
You can think yourself about it: how do we make the same design of glove with a larger hand compartment without also ending up using more materials/padding and therefore increasing the weight?
I read an article by John Golomb awhile ago, talking about how Everlast made gloves for Liston ahead of one of his fights with Ali. Liston's hands were too big for the 8oz gloves that Everlast produced. The fight gloves had to weigh 8oz. They couldn't just use the same glove design and magically make the hand compartment bigger in it for him without also using more leather and more padding and therefore increasing the weight beyond 8oz. They had to remove some of horsehair inside and change it with a lighter foam to keep the weight down. That was apparently the first time that foam had been used in a professional fight glove.
There are gloves on the market that come with sizes S/M/L/XL etc, they're marketed as bag gloves usually (though not always). The larger sized gloves will weigh more than the smaller sized gloves, even though the manufacturer isn't making them with specific weights in mind. You can sometimes get 10oz fight gloves with a normal and then a L/XL sizing but I rarely see that with 8oz, and to accomplish that there will be slight differences in the design, which would become pronounced and obvious if you tried to have a 16oz glove with significantly different hand size options.