Personally, I think there's a 5 month age difference between Kovu and Kiara. I reckon Kovu was about as old as baby Simba at the start of
The Lion King when Simba returned from his self-exile. This would be old enough for Scar to judge if he is physically fit to be his successor. So, maybe, he would be six weeks old, perhaps.
I've also heard lionesses are pregnant with a litter of cubs for 3 months. From up to Simba's ascension to being king and the birth of Fluffy/Kiara, I would estimate at least 5 months passed during the replenishing of the Pridelands. So, Nala's pregnancy with Kiara could fit easily into that period of time. 6-7 weeks after Kiara is born, she is presented, and by that time, Kovu would a six-month old cub.
Another six months pass for Kiara to be a full-grown lion cub, and Kovu, being 5-6 months older, is close to being a year old. Remember cub Kovu is older than cub Simba evident by his thin ear rims, which indicates he is starting to mature into an adolescent. Finally, the only reason I can think of as why Kovu is undersized is because of the lack of food and water in the Outlands as explained by Zira. So, Kovu barely has anything in his stomach to support his bodily system.