If it’s warm, you are only going to need one pair of long pants along with one sweater for the evening breeze. If you wear these on the airplane, you’ll be affording yourself much more room within the bag.
Layering while flying is also a great idea, as sometimes the travel clothing we wear during the trip leaves us either too hot or too cold while in flight. If you have on a t-shirt and a sweater you can easily remove the latter if you get too warm or put it back on if you feel chilly.
Smart planning is really all it takes. By mixing and matching and choosing easy to care for fabrics, your suitcase will be lighter and you’ll worry much less about what to wear while you are away.