Here is a composite picture showing Trig at the Mat-Su hospital with Sally Heath, and then at the Kristan Cole party.
I want to point out that there is no objective way to reach the conclusion that Trig in the right-hand photo is smaller than in the other photo. There is not a single common element except for Trig's face. So why does everyone get the impression Trig is smaller in the right-hand photo? Several factors contribute, but the chief factor, I believe, is that the right-hand photo was apparently taken with a wide-angle lens. (H/T to astute reader Susan for helping me figure that out.) Wide-angle lenses have the distorting effect of making closer things appear larger than they should, with the opposite effect for things that are further away. Here's a fun example I found at Flickr:
The photographer included technical information, including that the focal length of the lens was 22mm (anything below, say, 35mm will likely have a discernible effect, and the lower the focal length, the greater the effect). The distorting effect is obvious in this photo: the baby's head dwarfs his legs. (I used a ripple filter over the face, in case that baby reads this blog.)
Two more things contribute to the impression that Trig to the left is bigger. First, even though we know the baby does not have to fill every inch of that big bundle of blankets and whatnot in Sally's arm, the large size of the bundle suggests the baby must be large. (Who knows, maybe the baby had a lot of wrapping around it because he had been on a car trip that day.) Second, the picture to the left shows a nice chubby baby face because it's a straight on shot; the other photo shows a face that looks much skinnier – and even though we know intellectually that is largely because of the different angle of the shot, maybe our gut reaction is to say chubby-face Trig must be bigger than thin-face Trig. The wide-angle effect probably also contributes how we process those relative face widths.
I teach photojournalism, so I have a sense of how different lenses affect shots. But working backwards – looking at a given picture and deducing what kind of lens was used – can be tricky for a nonprofessional photographer like me. (I did take photographs when I was a working journalist, but I was mainly a reporter.) So for those of you who have a photography background, please weigh in and let me know if you think I got it right in this analysis.