The two sizes are different components rather than one that scales, because
the large chart shows strictly more: a y-axis, the session mean ± std band,
per-rep bars and a best-rep callout. Enlarging is therefore a swap, and the
caller keeps ONE expandedMetricID so only one metric is large at a time.
On enlarging, the item scrolls itself just into view — the large chart is roughly twice the height of the card it replaced, so on a phone the thing the coach just tapped would otherwise grow off the bottom of the screen.
One metric in a Performance grid, at either size: the compact PerformanceMetricCard by default, or — once the coach taps its enlarge control — the same PerformanceHeroChart the tab's primary focus metric gets, spanning the full grid width.