An ultra-thin movement is not a smaller movement. It is the same set of problems solved in less space, with each solution costing something elsewhere.
The barrel
A thinner barrel holds a shorter mainspring, and a shorter mainspring holds less power. Most ultra-thin calibres accept a reduced reserve rather than compromise the escapement.
The bridges
Thinner bridges flex. Flex changes pivot alignment, and misaligned pivots cost amplitude. This is why ultra-thin watches are more sensitive to shock than their thicker siblings.
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