A size constraint forces creative solutions to problems where the existing solutions are large. But that has nothing to do with conceiving of a novel mechanic. On the contrary, most creative processes start from a base of knowledge and skills, and build from there, often contrasting existing problems and solutions, brainstorming ideas until something novel arises. Size constraints would stifle that part of the creative process, not enhance it.
After you come up with an idea, evaluate it, and decide it is good to pursue, optimization makes sense. But no, don't put that at the beginning of the process.