The gazelle-like 21-year-old, who is 1.74 metres tall and weighs 64.5 kilograms, has signed a five-year contract with the Spanish champions, who paid 57 million euros for his services.
After competing for Brazil on home soil at the Confederations Cup, he is likely to make his Barca debut in a friendly in Gdansk, Poland, next month.
"We gave him a very speedy checkup and his weight was one of the pieces of information," Barca doctor Ricard Pruna told a news conference.
"The first impression is that his weight of 64.5 kilograms is on the low side and maybe he is lacking the odd little kilo but I cannot say exactly how much," he added.
"As the season progresses we will get to know the player and we are constantly gathering information. That way we can determine his ideal weight."
Neymar is not the first Barca player to be deemed too light. The club famously paid for their Argentina forward Lionel Messi, the four-times World Player of the Year, to have treatment to speed up his growth when he was a delicate teenager.
Pruna also said that surgery to remove a cyst from the knee of captain Carles Puyol on Wednesday was successful and the centre-back should be ready to begin pre-season training with his team mates in mid-July.
