Pokemon-go – Why is there a difference between the height/weight in the Pokédex and Storage

Update: It appears that the conclusion below is false, and as of now we are unsure of the correlation between Weight/Height and any other of the Pokémon's stats. I have seen several counter-examples to the trend I observed, both from personal experience and from other people and so I am no longer confident that there is a relationship. Once more information, incorporating moveset and other variables (such as - "was it caught at this CP level or did you raise it to this CP level?"), comes to light, hopefully we will have a more definitive explanation for the variability of HP vs CP.

The tests I conducted showed that yes, weight (and possibly height) are relevant to a Pokémon's stats. From what I saw, for two Pokémon of the same CP, the one with higher weight will have higher HP. This means that it is more important to hold on to Heavier (XL) Pokémon and place less emphasis on raising XS Pokémon. I did not focus on height when I pulled my sample together, but there is no clear trend across the two examples I used, its effects will need to be looked into further.

The two separate cases I used to come to this conclusion are as follows-

First Case:

I captured two Pidgeys of the same CP level (93) in a very short time. Comparing their weights, I found that one had higher HP, and realized that this was positively correlated with weight. Interestingly, in this case the Pidgey with the larger height value was the one with less HP, which I thought may thought might indicate a negative correlation, but this was not consistent with the next case.

Second Case:

I had one sparrow of CP level 77 and another at a lower CP level, but with an (XL) tag for it's weight. I upgraded the lower one to CP 75, and discovered that the (XL) Pokémon had more health than its smaller cousin (who was two CP levels higher). This certainly offers more support in favor of weight factoring into a Pokémon's HP. Interestingly in this example the height increased along with the HP/weight, arguing against a negative correlation.

The amounts shown for HP differences may seem small, but keep in mind that this is at very low CP levels, so I expect the effect to be more noticeable at higher levels.

Obviously this is only two cases, so drawing conclusions based off of such a small sample size isn't ideal, and I will try to find more examples that can help shed light on this relationship. If anyone has any contradictory examples please post them, as it would definitely help clear this up.

Edit: I'll post the additional cases tomorrow, but I have found four more examples where a higher weight Pokémon of equal or lesser CP has a higher HP than its smaller cousin.

In terms of actual numbers (as in, how many kg for XL and what's the absolute max), there is not yet enough information to give a solid answer. Remember the game came out recently (at the time of writing), and therefore little information is available.

However, Pokemon are rated on a scale of XS to XL (with others just not being recorded). Pokemon of any species can fall anywhere within this range, so an XS Charizard is just as easy as an XL Weedle.