Why your icon viewer only ever shows one size
Open a .ico file in a typical image viewer and you'll see a single icon. That's misleading about what the file actually contains: an .ico is a container format that bundles multiple resolutions of the same icon into one file — commonly 16×16, 32×32, and 48×48, sometimes larger — so that whatever's displaying it (a browser tab, a bookmark bar, a desktop shortcut) can pick the size that actually fits, rather than scaling one image up or down and losing sharpness.
Most viewers just show you the first or largest entry and quietly hide the rest, which is exactly why a dedicated .ico inspector is useful the moment you actually need to know what's packed inside — checking a favicon before deploying it, verifying a client-provided icon file has enough size variants, or debugging why a site's tab icon looks blurry at one specific size.
Two icon encodings living inside the same file format
Here's a detail that trips people up: not every entry inside an .ico file is stored the same way. Larger, more modern icon sizes are typically PNG-encoded — modern compression, previews cleanly in any tool that understands the format. Smaller, legacy sizes are frequently stored using the original ICO specification's raw bitmap format instead, a decades-old encoding that predates PNG entirely and needs more specialized decoding to render visually.
This is why a good .ico inspection tool shows a live preview for the PNG-encoded entries but lists size and bit-depth metadata (without a visual render) for the raw-bitmap ones — it's not a limitation of the tool so much as an accurate reflection of how mixed the format actually is internally.
Why this matters for building a proper favicon
A well-built favicon.ico bundling 3-5 sizes covers every context it needs to: a small size for a crowded browser tab bar, a mid-size for bookmarks, a larger size for a desktop shortcut or high-DPI display. Ship an .ico with only one embedded size, and you'll get a blurry, upscaled or downscaled icon in at least some of those contexts, even though the file technically "has a favicon." Checking what's actually bundled before deploying catches this before a user ever notices.
Pulling out just the one size you actually need
Sometimes you don't want the whole bundle — you just need one specific resolution as a standalone file, say for a design mockup or a different platform's icon requirement. For any PNG-encoded entry, once you can see it rendered as a live preview, saving that single image directly gives you exactly that one size on its own, without needing separate software to unpack the full .ico container.