Answer:
It is mostly down to body size. Great Apes (gorillas, bonobos, chimpanzees, orangutans, humans) are simply that much bigger than the Lesser Apes (gibbons and siamangs).
Great Apes are considered to be more intelligent than the Lesser Apes, and there have been various experiments to show this. Gibbons have not been shown to be able to determine their reflection in mirrors (but it seems to depend on the individual in the Great Apes, since not all individuals can do it!). They were also considered to be unable to use tools, however this was a fault in the experimental method, as Gibbons have considerably longer arms. Once the experimental design took this into account, they did just fine!