On pages 66 to 67 of Rossabi's Khubilai Khan: His Life and Times, he has this to say of the portrait and of a later portrait in 1280 of a hunt, also seen in the English Wikipedia article for Kublai Khan^[2]:

A Chinese portrait of him painted around this time [i.e. roughly the time of the rebellion of Li Tang and execution of Wang Wentong in 1262] shows a robust, determined man. He wears a simple white cloth garment; no silks or furs adorn his body. His black and white hat is hardly lavish, and his mustache and beard are trim and obviously cared for. Most important, the picture shows that Khubilai had not yet abandoned himself to sensual pleasures. Though certainly not gaunt, neither was he obese, as he became toward the end of his reign. Probably food, Chinese or any other kind, had not yet become a consuming passion; nor does he show any sign of being a heavy drinker, as later he would become. His alertness and robustness contrast sharply with his appearance in a painting executed in 1280. Two decades after assuming power in China, he had become grotesquely fat.