Yes I'm always a bit dumbfounded by this behavior as well. They always use weird stuff and I never have the intuition of the actual size, especially since the definition can vary depending on context.

In this case, what is actually considered to be a small dog? To me it would be something that is close to the size of a cat but since it's about 13kg, it can't be that small, so that's more like a medium dog (I'm not certain, but I have a feeling that if you lay out things statistically this is what you would end up with). On the other hand, 13kg is very easy to get, that's just 13 liters of water, and it's quite easy to make a mental image for both volume and weight "feeling" that way.

American units feel so impressive and random, it is the reason they always add those weird comparisons but often they make it even worse.