Bend it more, use all 3 dimensions for these bends. Mold it in high dielectric material.
Any volume reduction comes at a cost. Efficiency and/or bandwidth will be reduced.
you can use series inductors and shunt capacitors to resonate it with smaller lengths of line. Like mentioned, it you do that aggressively, efficiency will suffer. There is no such thing as a free lunch
Please search for paper of Kin Lu Wong he studied difference for of PIFA antenna at more than 100 paper and he notice to miniaturization also u can find His book with title of Planar antenna that define every method clearly