Look at your own list here - Yan is literally the same height or taller than MOST of his opponents, including the current and former champion.
If your argument is that he has average reach for his height, sure. He is not the size of a flyweight and he's not a small bantamweight.
He is lean and muscled on the scale and re-hydrated 10% of his bodyweight, which was MORE THAN AVERAGE across all divisions on his CSAC fight card.
This isn't an opinion, its patently absurd to say Yan is a overly muscled flyweight, stupid to the point of hilarity I'm even typing this but here we are.
Height difference is pretty meaningless when it's an inch. Reach is a far more important factor, especially when you are a striker that prefers to box. Let's look at averages.
-Of the 14 opponents he fought the average height is 5'7'', so he's of average height based on opponents (if it was the current top 15 the average would be 5'8.5'', the division has gotten noticeably taller/longer in the last few years).
-Of the 14 opponents he fought the average reach is almost 69 inches (68.7), so he's giving away an average of 2 inches of reach to opponents (if it was the current top 15 the average would be like 69.5 inches).
-Of the 9 FLY I listed (9 of the current top 15) the average height is 5'7", so he's basically as tall as the majority of elite FLY.
-Of the 9 FLY I listed the average reach is almost 69 inches (68.88), so he's giving away an average of 2 inches of reach to the majority of elite FLY.
The difference between being a FLY and a BW build is primarily how thick/muscular you are, but the athletic frame intangibles are how tall/long you are. 90% of elite BW have more reach, 65% of elite FLY have more reach.
He's too thick/muscular to cut the weight to make FLY, but he has the height of the average elite FLY and he has less reach than the average elite FLY. If he hypothetically could make the weight to FLY he'd have no height advantage against most of the top 15 and would be at a reach disadvantage against the majority of the top 15.