Imperialism doesn't always require the actual taking or claiming of land. US Imperialism can include things like McDonalds and Starbucks on every street (cultural imperialism). It can include things like bringing a region of countries under your political control so they'll be more likely to support your wider positions in things like the UN (think the Cold War and the First and Second World countries). Throwing your weight around to expand your own personal power in the global world falls under this.

Of course this can absolutely be achieved directly through expansionism, colonialism, invasion, coups, etc. So there are wavelengths where these things do match up in some sort of evil Venn Diagram, but there are still distinct factors which can separate the concepts to keep them from being one-to-one synonyms.

If I were to put it as briefly as possible: Expansionism is the means, Imperialism is the effect that is had on the affected people, the culture, etc. and the benefits given to the driving-power after the fact.