Maybe someone could comment on my hypothesis:

Is it possible that technological innovation is a direct factor of how many people are alive at a given time how specialized their roles are (eg they do more than sustanance work), combined with the ability to store and transmit that data. The simple raw brain power directed at any given problem can usually brute force an answer over time.

For technological innovation the three most important factors are storage and transmittance of data, and processing power.

None of these factors can be met in small scale hunter gather societies in any meaningful way. Storage is that which is held in the mind of living members, transmittance is through word of mouth, processing power is limited to the size of the tribe probably less than several hundred individuals on the high end. Trade of knowledge between tribes is somewhat rare as well.

Small scale agricultural societies are little better still small numbers of people passing knowledge from person to person through word of mouth without any kind of storage. However in agricultural societies personal (especially property related) rights begin to grow beyond those of communal rights; after all if you have worked your field for the year you don't want someone else reaping the benefits of it. These personal rights necessitate recording of ownership and other rules which creates unified systems of data recording.

Systems of recording allow data transfer across multiple generations, and solves the issue of transmittance and storage, knowledge is no longer lost when a person dies (or is less often lost), and even in large scale epidemic can be recalled to some extent. Processing power is still an issue.

However as populations grow there is likely to be individuals who solve major problems these solutions this information can be past on. Some of this information will be important for population growth. Population growth can cause an increase in specialization of individuals which will allow new problems to be solved more quickly as more humans (processing power) is brought to bare on them. Which causes a feed back loop where more people = more innovation which = more people. For example anti-bionics were discovered quite accidentally but are a major cause of the population boom of the last century, which has seen a change in technology that dwarfs preceding hundreds and thousands of years.