One thinks he had a big ego and was overtly optimistic.
It's an interesting question. Light was quite tightly controlled by the Commissioners for Colonisation. Remember that he was an employee - a military surveyor hired to produce as quickly as possible a survey of the city of Adelaide and enough country sections to make possible Wakefield's model of settlement which involved auctions of 80 acre country sections to each of which were attached one of 1042 city acres.
I think everyone involved was optimistic. No-one knew how successful the colony would be, and my guess is that they decided 'better too large than too small'.
As to Light's character: When Light was hired, he was on the Nile in the yacht 'Gulnare' which belonged to his second wife, an heiress from whom he had separated during a cruise of the Mediterranean. Between 1830 and 1835 Light was working for Muhammad Ali, the ruler of Egypt, remodelling the Egyptian navy, when he was asked to go to London. Also in Egypt, working under Light, was Captain John Hindmarsh, who also went to London. There was some argument among the people trying to organise the new colony about whether Light or Hindmarsh should be the first governor. Hindmarsh, who disliked Light, won.
Light spent some weeks being 'schooled up' on the requirements for the new colony by the organisers, including being advised about the ideas of the philosopher Jeremy Bentham and others about the form of the ideal city.
Light managed the survey of the town acres very quickly, doing it on foot because there were no horses in the colony. He developed a method, innovative at the time, for surveying the country sections by triangulation from trig points, meaning he didn't have to physically traverse the entire landscape.
Light was by all accounts an extremely thorough and efficient engineer and surveyor. He was also good company, it seems. He was an aide-de-camp to the Duke of Wellington, who defeated Napoleon at Waterloo, and was valuable in that job not just for his efficiency but for his ability to play the guitar and dance. Light kept Wellington's HQ full of girls and generally ensured everyone had a good time.
By the time he arrived in Australia, Light had his new mistress with him - Maria Gandy, who had sailed with her brothers to Australia on Light's ship Rapid.
By all accounts, Light was a good bloke - friendly but firm about what he thought was right, as indicated in his 'epitaph':
'The reasons that led me to fix Adelaide where it is I do not expect to be generally understood or calmly judged of at present. My enemies however, by disputing their validity in every particular, have done me the good service of fixing the whole of the responsibility upon me. I am perfectly willing to bear it, and I leave it to posterity and not to them, to decide whether I am entitled to praise or to blame.'
The above should give you some idea of his character as I have found it at least, in reading the history of SA. Here's a picture of Light when he was on Wellington's staff.