ŝveli

One root, no joints, and nothing to find inside it. Ŝveli looks as though it might start with something, because Esperanto really does have a lot of one-syllable pieces that clip onto the front of words. It has no ŝ- prefix. There is no root vel- here either, though velo, a sail, exists as a separate word and is unrelated. Ŝvel- is a single Germanic root, the same one that gives English swell, and it comes in whole.

Now the grammar, which is the part that actually catches people. Ŝveli is intransitive: a thing swells by itself, and it takes no object. Lia genuo ŝvelis is correct; li ŝvelis la genuon is not Esperanto. To make something swell you need the causative -ig-: ŝveligi la balonon, to blow the balloon up. And ŝveliĝi is heard, but it is redundant, because the plain verb is already the intransitive one. If you find yourself writing ŝveliĝi, the word you wanted was ŝveli.

The family and the near neighbours. Ŝvelo is a swelling as a thing you can point at, ŝvela describes something swollen, ŝveligi is to inflate, disŝveli is to swell apart or to bulge. Kreski is to grow, which is what a plant or a child does over time; ŝveli is what a knee does overnight, and the difference is that swelling is a change of volume rather than of size in the healthy sense. The figurative use is available too and works as it does in English: a river swells, a crowd swells, and so does a heart.