Not Heermann's Gulls. They are more solid, sooty gray-black. The reason these birds appear slim in this photo is because the bill of the bird on top is viewed from above, and gull bills are thinnest in width so appear thin when viewed from above. Also, the lower bird has its bill open, and the lower mandible is hidden by the fish, so only half of the bill is visible. Given how dark these birds look, I'd say they look good for young Western Gull.