Many people only pay attention to the weight of a sleeping bag, but is that really the most important thing?

Not at all! After all, a sleeping bag is for sleep. If it's cut too small just to save weight, it may be easy to carry, but it won't provide a good night's sleep. If you sleep poorly, you won't have the energy to carry even a lighter sleeping bag the next day. The extra 100 grams is ultimately a small detail compared to the importance of restful sleep. Sleep should be a priority.