Earlier today a well-known community tester of USB devices reported that the 5" version of Google's Pixel phones was not able to pull more than 15 watts of power off the stock 18W USB-PD charger, despite Google's specification site for the Pixels implying heavily that both devices were capable of 18W charging.

Charging USB Type-C™ 18W adaptor with USB-PD 18W charging

We reached out to Google, and it turns out this was just a marketing materials mistake: the official specification page for the Pixel has been updated to correctly state that the phones support USB-PD charging from 15-18 watts, as opposed to simply 18 watts. The updated page, now live, reads:

Charging USB Type-C™ 18W adaptor with USB-PD 15W - 18W charging

Nathan speculates in his Google+ post that it seems Google may have reduced the maximum charging wattage on the smaller Pixel out of thermal concerns, though Google did not comment on their specific reasoning for the different charging speeds. The variance between the two devices accounts for a little over a 20% speed boost (on paper) at peak charging speeds for the XL compared to the standard 5" Pixel. However, the XL also has a battery around 20% larger, so it actually seems likely the XL probably still takes a bit longer to charge.

The reason I say "on paper" and "peak" is because a smartphone will not always utilize its maximum charging speed depending on conditions of temperature, whether the display is on, and the current capacity of the battery. All phones slow down charging very considerably as their lithium batteries near capacity (probably around 80% is where things start to crawl slower and slower) simply by the nature of the limitations of the battery chemistry. As such, while "20% faster" sounds like a lot on paper for the XL, the difference in 0-100% times between the two phones is realistically not going to be affected all that much by a gap of 3W at the peak charge rate, particularly given how much smaller the Pixel's battery is than the XL's.

If you want a sidebar opinion on this: my guess is that based on Nathan's post and Google's subsequent correction, the Pixel team may have adjusted the 5" model's maximum charging speed down some time late in development or right before the firmware reached final, and the marketing team just never got the memo.

If you feel this was a major omission by Google, well, I'd say you at least have a reason to return your phone in principle - Google did technically falsely advertise the peak charging speed of the smaller phone. But in reality, this is a fairly minor change to the product specification, and one that Google has admitted was more of an oversight, and not some major product change they were reluctant to acknowledge. I highly doubt Google intentionally misled anyone here, and they're making the situation right, immediately confirming Nathan's findings as accurate.