Growth outward into more room than before. Expansion adds reach and takes something to sustain.
Latin expandere, ex- (out) plus pandere (to spread out); the noun for the spreading-out.
Its root also gives spawn, by way of Old French espandre: both words name a spreading-out from one point into many. Expansion is that same spread applied to room rather than to offspring, and it costs the same thing either way — more to sustain everywhere it now reaches.