I have several packs, too. A small daypack for summer dayhikes and frontcountry (state parks, etc.) hikes. It's the old 24 pack from Golite. I can carry rain gear, water, lunch, the usual ten essentials stuff, but not much more.

Next up is an Osprey Kestrel 32, which is a framed day pack that can easily carry 30 pounds. It's used for photo equipment, birding stuff (scope and tripod), long backcountry dayhikes, travel, and even a light overnight. Just got it last year, and I love it.

My backpacking pack has been the Six Moons Designs Starlite, which is a terrific long distance hiking pack. Lots of room, very comfortable, and under two pounds. But it's far too large for day hikes -- it's just not comfortable with tiny loads.

I still have my old Gregory Shasta, 6000 cubic inches of load monster pack. Over six pounds, and that's after cutting off every extraneous strap. I haven't carried it in probably ten years.

To answer your question, Yes, I would buy a day pack specifically for day hikes. I can recommend the Kestrel.