Size is irrelevant to whether a country is being imperialist or not.
I'd argue that regional powers' imperialism is even worse than that of global powers as a general rule since regional powers' imperialism has an element of cultural and social-imperialism coming with it and not only lasts longer but also have a larger presence in the target country due to the close distance, while a country like the US or China are far less attached with their imperialism being limited to resources and access without any significant cultural and social elements.
Also, this just in:
Black is white, and night is day.
*Of course* size matters, because a larger military empire means more military coercion *worldwide*.
You seem to measure 'imperialism' in terms of *culture*, for whatever odd reason.
US influence and voting reform
The scholarly consensus is that IMF decision-making is not simply technocratic, but also guided by political and economic concerns. The United States is the IMF's most powerful member, and its influence reaches even into decision-making concerning individual loan agreements. The United States has historically been openly opposed to losing what Treasury Secretary Jacob Lew described in 2015 as its "leadership role" at the IMF, and the United States' "ability to shape international norms and practices".
Reforms to give more powers to emerging economies were agreed by the G20 in 2010. The reforms could not pass, however, until they were ratified by the US Congress, since 85% of the Fund's voting power was required for the reforms to take effect, and the Americans held more than 16% of voting power at the time. After repeated criticism, the United States finally ratified the voting reforms at the end of 2015. The OECD countries maintained their overwhelming majority of voting share, and the United States in particular retained its share at over 16%.