We've seen both, we've played with both, and here we've pulled apart the specs of both devices to answer that question and determine exactly what the difference is.
1. Design and build
Both the HTC One mini and the HTC One offer great built quality. They feel solid in the hand, they look great, they come with a premium finish. The HTC One mini has a plastic band that runs around the edges, whereas the HTC One's is inset, but apart from that, the design is very similar. The HTC One measures 132 x 63.2 x 9.25mm, the HTC One measures 137.4 x 68.2 x 9.3mm; the Mini weighs 122g, the original One is 143g. The HTC One mini certainly lives up to its name.
Of course one of the reasons that the HTC One mini is smaller, is because it has a smaller display. The HTC One mini display is 1280 x 720 pixels, spread across a 4.3-inch screen. That gives it 341 pixels per inch. The HTC One on the other hand is 4.7-inches, with a 1920 x 1080 pixels, a cracking 468ppi. The HTC One is sharper, it will give you more detail in the same space, but both are bright and vibrant, both have plenty of punch and great viewing angles.
This is where the HTC One revs its engine. The HTC One mini has a 1.4GHz dual-core Qualcomm Snapdragon 400 chipset, the HTC One has a quad-core 1.7GHz Qualcomm Snapdragon 600 chipset. In addition, the Mini only offers 1GB of RAM, which the HTC One doubles with 2GHz. No doubt about it, the original HTC One has more grunt for those more intensive apps. It also has double the storage at 32GB compared to 16GB.
Smartphone camera seem to get all the attention and both the HTC One mini and the HTC One feature the 4.1-megapixel UltraPixel camera on the rear. This is supported by a range of smart shooting features, like video highlights and Zoe. The sensor is the same, but the HTC One mini doesn't have optical image stabilisation. It also takes a hit on the front camera: the Mini gives you a 1.6-megapixel sensor, the HTC One a great 2.1MP wide-angle snapper.
5. Wireless hardware
There are a few other bits that are different on the hardware front. While both handsets offer you LTE, Bluetooth, GPS, it seems that the Mini misses out on 802.11ac support. Faster Wi-Fi props go to the HTC One, as does NFC and the IR blaster built into the power button.
Does size matter?
There are several areas where these two HTC handsets differ, but there's also a huge range of similarity. Importantly, both are running Android 4.2.2 with HTC Sense. Both look, feel and behave the same way when you interact with them, so the user experience is very close.
The long and short of it is that the original HTC One will do everything and more that the HTC One mini will: it just costs you a little more and will put a bigger bulge in your pocket.