I imagine Ni to be about changing perspectives. From this perspective or that, two things do or don't look similar. Changing a perspective brings to light previously hidden connections. But these words--"change", "look", "light", "hidden"--suggest a physical limitation that is inappropriate. Actual Ni perspective shift is a change of the framework and terms under which the "objects" are defined. (And they are "objects" because, at least for NJs, the intuition is working with an objectifying e function.) The degree to which any Ni user is any good at Ni use is the degree to which they can invent or discover substantial new frameworks and terms.
Si, I think, does something very similar. The content differs substantially, however. Also I think perhaps the Si user is not bent exactly on inventing and discovering new frameworks and terms, but on acknowledging and consolidating frameworks and terms. A small difference. They build from the detail up.
(I just made up both those sets of claims. They may not be true.)