I think these two factors (weight and flexibility) are important to be considered. You must use a considerably light material with high level of ductility. Nonetheless the material must be tough enough to withstand the predetermined load. I suggest composite material or aluminium alloy. Combination of Casting and welding processes should be the most appropriate.

Dear Syahmi Uzair, the opinion of Shuaibu Ochetengwu Yakubu is reasonable. The requirement of maximal size decreasing may be satisfy if we use transformer-like design of this chair. Such construction assumes the application of reliable joints and bars which were developed in robotics (manipulators, exoskeletons) and prostheses (artificial legs and hands) for medical needs.

Good luck!