I generally only carry enough to [almost] fill my TB20 or B25. When I carry my K35 or other larger formats (I have a large Fendi Peekaboo and a Faure Le Page 32cm tote), they're generally pretty empty, but they fit the laptop nicely (well my current laptop is technically a bit too big for the K35 but it gets stuffed in sometimes anyway). Honestly, whatever makes you happy...a small bag overflowing or a large empty bag, whatever who cares it's your life to enjoy 
