This paper presents a study on the effect of aggregate size on spalling of concrete in fire. Three concretes with completely different chemical compositions and strengths were investigated, namely, geopolymer concrete and high strength and normal strength Portland cement concretes. The effect of aggregate size was found to be the same regardless of the type of concrete. The concretes containing 10 mm aggregates spalled while the ones with 14 mm did not spall. Since this effect is the same in geopolymer and Portland cement concretes, it is independent of binders chemical compositions. This paper shows that the degree of spalling has a good correlation to the fracture process zone length, which increases with increasing aggregate size; this in turn reduces the flux of kinetic energy from pore pressure and thermal stress that is released into the fracture front and thereby improves the spatting resistance.