Arm protection from the sun
They're basically arm warmers. They have to be close fitting to stay in place. I suspect you're not sweating as much more as you think, but it's not evaporating as quickly - her way you can easily end up sweatier.

Some arm warmers/sun protectors are better than others - thinner and better at wicking sweat away. So you may have been a bit unlucky. Light colours are also helpful in the sun, but I'd rather use suncream even if that means carrying some and topping it up. I found that using sleeves like yours as sun protection left me with little gaps that kept opening up where they met my jersey sleeves, and I got sunburnt there. Yours may fit better.