The Marxist definition of imperialism is more specific than just "big country invade small country".

In, Imperialism: The Highest Stage of Capitalism Lenin lays out five aspects of what makes Imperialism:

1. the concentration of production and capital has developed to such a high stage that it has created monopolies which play a decisive role in economic life;

2. the merging of bank capital with industrial capital, and the creation, on the basis of this “finance capital”, of a financial oligarchy;

3. the export of capital as distinguished from the export of commodities acquires exceptional importance;

4. the formation of international monopolist capitalist associations which share the world among themselves, and

5. the territorial division of the whole world among the biggest capitalist powers is completed. Imperialism is capitalism at that stage of development at which the dominance of monopolies and finance capital is established; in which the export of capital has acquired pronounced importance; in which the division of the world among the international trusts has begun, in which the division of all territories of the globe among the biggest capitalist powers has been completed.

The question of "Is Russia Imperialist" isn't a moral one, it's a technical one. So if Russia were do to something that we all agree is morally reprehensible, that's a separate concern from whether Russia is imperialist.

The technicality revolves around whether Russia has developed an oligarchy of Financial Capital, such that its invasion of Ukraine or other flexes of its influence, perpetuates the export of Russian finance capital around the world.

As it stands now, I don't think that's currently the case, but with Marxism being a dialectal philosophy, I do wonder if this war will accelerate that merging of Bank and industrial capital that Lenin discusses. It's a Bourgeois states, and there's financial capital in there somewhere that absolutely has an interest in forming a Russian imperialism.

So when people say "Russia isn't Imperialist", this is what's being referred to. You can take it or leave it, but it's worth getting into the weeds a bit, so we aren't all talking passed each other