My pencil case priorities have shifted. I used to think more room was always useful, but a bulky case is less appealing when I only carry a few pens day to day. I'd rather have one that's easy to slip into my bag than space for supplies that mostly stay at home.
Highlighters are where I can accidentally turn a slim case into a rainbow stash. I stick to two colors for whatever I'm working on, so the color key still works without hauling the whole set.