Let me quote these lines from wikia:
The Rasengan does not require hand seals to perform. Once it is
formed, it also does not require any additional chakra to sustain it
Although it doesn't require any additional chakra to sustain it (so I guess once it is formed it can be used by another person, too), but that doesn't also mean that it will not be difficult; it will be difficult to handle that chakra and to perfectly balance it.
In case of Boruto's Rasengan:
Though Boruto's Rasengan is initially much smaller, he subconsciously
applies wind-natured chakra to it, enabling him to hurl the
Rasengan across distances. While it loses its physical form shortly
after bring thrown, the wind and force continue on-course unseen,
tricking the opponent to let their guard down as sufficient damage is
inflicted when it makes contact.
At first, Boruto really tried hard to achieve this stage so there is no doubt that he won't be able to handle Rasengan. But there is one more point to note: Boruto subconsciously applies wind-natured chakra to his Rasengan which perfectly defines that he was able to create Rasengan with that much power.
At the end of the movie:
Naruto is impressed that Boruto was able to learn the Rasengan and,
still unable to move, adds his own chakra to Boruto's Rasengan, making
So Naruto simply added his own chakra to make that Rasengan much bigger. Since Boruto also learns how to perform Rasengan while training from Konohamaru Sarutobi after extensive practice then that's why he was able to handle Bigger Rasengan. (Note: that it was his own Rasengan but in bigger size than the usual one.)
Also in Boruto Uzumaki's wikia page, there is nothing mentioned about Choodama Rasengan (also in movie) in the Jutsu section, so according to the given information, we must accept the fact that it was not a Choodama Rasengan.