They certainly can be. Logically, they are bigger than single strollers so therefore will be heavier.
Let’s look at an easy comparison. The Kolcraft Cloud Plus single stroller weighs 5.8kg (12.8lbs). The Kolcraft Cloud Plus double stroller is nearly twice as heavy at 10.4kg (23lbs).
However, you can definitely get ones that are lightweight for their size. The Kolcraft Cloud Plus is itself one of the lightest double strollers on the market, along with the Baby Jogger City Tour 2 Double. And there’s the ultra lightweight, no frills Delta Children LX Side by Side which weighs just 8.9kg (19lbs).