There is no such thing as ambition, what does exist is contentment/personal satisfaction. I have a theory about what determines a person's 'ambition' as regards work they do. I don't have a great description of it yet, but roughly:
What is referred to as 'ambition', how challenging work one does is, is only one part of several factors making up a total personal satisfaction of a person. I refer to total personal satisfaction as their sweet satisfaction zone (SSZ) which exists on a spectrum of satisfaction (SoS). Everyone's SSZ, to which several factors contribute, determines an individual's mental state in the long term.
Among the factors affecting where their SSZ lies on the SoS are things like the challenge of the work they do, their relationships with friends, their romantic relationship(s), their status in the hierarchy of their local community etc. Everyone requires different levels of satisfaction from each of these different factors depending on their personality, but each of these factors contribute to the SSZ.
When some people work on what seems 'ambitious' to some others, it may just be work that feels sufficiently challenging to them, as they require a huge amount of satisfaction from interesting and high-impact work to contribute to their SSZ. And when a person one thinks to be clever and capable of more works on what one thinks to be below their intellectual ability, it may be that how challenging work they do is doesn't rank on factors they demand huge amounts of satisfaction from to make their SSZ. The golden handcuffs are in fact personal satisfaction.
So 'Ambition' doesn't exist in the sense people like to talk about it. You cannot really be more ambitious or less. It is based on personality.