I agree with your assessment: the bottom right sprite doesn't quite match the rest. However, I think the fact that on average the clusters are smaller isn't itself the cause, but rather a symptom of an underlying problem: shading every bit independently of the whole. For example, why are the hip and shoulder so dark? The limbs would be protruding there and they would be connected to the torso, but shadows give the impression of gaps and dips, not protrusions. The other sprites don't have great forms, but they feel more cohesive than the dino.

Here's a quick edit where I tried to connect the disconnected forms a little more, and reduced the contrast on the head so that it's not so overwhelmed by its texture:

This edit doesn't go for consistency with the other sprites, but I think it does help a little.

As for values: you don't need to use the exact same values - after all, sometimes you want to depict lighter or darker objects! Rather, your black and white point should be consistent across sprites (even if some sprites don't get anywhere near those extremes), and your ramps should have roughly the same number and distribution of colours for a given value range.

The rat also stands out from the rest because it's a dark animal, but its black is lighter than the "black" on the other sprites.

An easy way to help consistency is to reuse colours across sprites. I like to have my ramps start and end in the same "black" and "white", and to link between ramps where possible. This will tend to unify the colours, and give me many opportunities to reuse colours even between sprites that have different overall hues.