1)Mercury is a third of the size of our planet.
I think it means that Mercury is one-third of Earth. How is the structure?
2)Two cities which are next to Kuala Lumpur in size are Tretes and Seleta.
What does it mean here ?
For example #1,
size here really means mass. Mercury's mean radius is
2,439.7±1.0 km, which is "0.3829 Earths" in size. If you imagine Earth as a tennis ball, then Mercury might be a golf ball. These sizes aren't exact, it's just so you can imagine the difference.
For example #2, "size" could be talking about physical size (mi2 or km2) or population size (millions of people). This example means that Tretes and Seleta are two cities that are around the same size as Kuala Lumpur, except we don't know if they mean around the same size in physical size or in population.