Ranjan is very energetic and is interested by life in the jungle. He seems to like tigers, shown by the fact that he continuously tries to roar like one. He also does not like it when he can't stay up past his bed time. When Mowgli showed Ranjan how to get a banana from a tree in a method he learned in the jungle, Ranjan was impressed and ate the banana in delight. But when Shanti showed Ranjan how to peel a mango using a technique she learned in the village, he was pleased even more and proceeded to eat the mango. Ranjan will also lie about his hunger when he wants to get something done quickly, as shown when wants to skip breakfast in order to beat Shanti to the river so that he could play a prank on her. Ranjan is shown to be chubby, meaning he either eats a lot or has his father's genetics.
Ranjan did not gain weight in "The Jungle Book 2". However, he did gain weight along with Mowgli and Baloo in a fanfic by comicfan67 of Deviant Art. The fanfic along with an illustration of it can be found here: http://mothman64.deviantart.com/art/Jungle-Book-Weight-Gain-302871695