Mitochondria and chloroplasts display similarities with bacteria that led to the endosymbiont theory. This theory states that an early ancestor of eukaryotic cells engulfed an oxygen using non-photosynthetic prokaryotic cell. Eventually, the engulfed cell formed a relationship with the host cell 1n which it was enclosed, becoming an endosymbiont (a cell living within another cell). Indeed, over the course of evolution, the host cell and its endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion. At least one of these cells may have then taken up a photosynthetic prokaryote, becoming the ancestor of eukaryotic cells that contain chloroplasts.
So my question is, why is it that all the cells that took in the chloroplasts became plants or other simple animals but not large mammalian or reptilian animals? Is there a limiting factor that restricted chloroplasts from being beneficial to large animals? Why is it that large plants can have both chloroplasts and mitochondria but not relatively larger animals?