Each card shows the icon at 14 px next to the “Running” badge (how it'll actually appear in the activity page), plus a 32 px preview so you can see the detail. Filled silhouettes only — outline icons lose too much stroke at 14 px for the shape to read as a turtle, and filled shapes carry more contrast area, which is friendlier for low-vision users.
Note: neither Tabler nor Phosphor ships a turtle glyph. Lucide has one, but it's outline-only — pulled from this comparison because at 14 px the outline doesn't read as a turtle.