...They are just the thin end of the shim...

I smiled as I read this - I assumed it was tongue-in-cheek. "Thin end of the wedge" is the common expression I am familiar with. Like wow's carpentry example, for me (mechanical engineering/machinery applications) a shim is a thin piece of metal used to adjust something to a critical position or size. A shim is usually VERY thin and uniform thickness, so the "thin end of the shim" is a nonsense.

[edited in later]... I've just done what I should have done before my post - looked up a dictionary. M-W has shim as:
a thin often tapered piece of material (as wood, metal, or stone) used to fill in space between things (as for support, leveling, or adjustment of fit)

Hmm, so it's not nonsensical or tongue-in-cheek as I thought.