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.
The semicircular bar behind a Pokemon is, in fact, linear.
The lower end of the bar represents some (still unknown) value dependent on the family of Pokemon, and the upper end of the bar represents that Pokemon's current CP cap at your level.
I went ahead and proved the linearality of the semicircle using GIMP. You may see the results here. Note that there may be some error in the numbers due to inaccuracy of my tests (things may not have been lined up quite right).
(source)
Best Answer
From the most recent APK datamine, Low Kick has a set damage number given and has a defined DPS. Due to this, it seems like Low Kick will not take Pokémon weight into account when determining damage:
I believe the fact that the weight part of the move description was left out also alludes to this being the case. This is not the first time a move with secondary effects in the main series has been missing its effect (moves that always cause a critical hit, moves that raise/lower stats, etc).