While I am not going to argue that valuations are wholly rational (and specifically the fact that valuations increase with investment size), it is also true tha

While I am not going to argue that valuations are wholly rational (and specifically the fact that valuations increase with investment size), it is also true that having more capital may make the company able to accomplish more, and thus raise the expected exit value for the investor. If so, that provides a rational basis for increasing the valuation of the company when giving it more capital. (Present valuation being the discounted future valuation)