I just looked at some numbers and you're right, it does seem like I exaggerated. If you choose the lower weight range for the 3 series though the difference ends up being 14% heavier, and obviously you can choose difference cars to compare. But you are right, 1/3 seems too high. Maybe half that would be more accurate.
I looked at the lighter weight BMW model 3 series. They were either smaller engines (not fair since the model 3 AWD is already faster), or RWD (again not fair to compare to an AWD electric).
For the Model S my first thought was the M4, which actually does end up with the original 1/3 weight disparity I mentioned, but I think this is just a coincidence of my first choice. Compared to an E63 AMG, a car in a similar category known for carrying a bit of weight, and the difference is closer to 10%. Compared to an RS4 the difference is about 20%.
For the Model X I was really surprised how small the differences were. Compared to a G Wagon, which I'd say is really its competition, the difference in weight is about 8%. Compared to an X5, it's about 5%. If you consider the X7 to be the competition (which most people probably do), the X7 ends up being about 1% heavier.
And just to add it since I'm already doing this, I think the clear competitor for the Model 3 Performance is the M2 Competition. They're specifically meant to be track vehicles that you can comfortably use for your daily driver. This puts the weight disparity at 18% for that model. And just for fun: https://youtu.be/HycUgd6fTWI & https://www.youtube.com/watch?v=9Pu9046wX9g
So I guess the conclusion is electric vehicles are heavier, but as you move towards already heavy vehicles like SUVs the weight differences start to vanish. But as you move towards more smaller vehicles, the difference is much more noticeable.
Come now, that's not even close. That's comparing a BMW M4 2 door coup with a rear seat (rated for 4 adults) with a fullsize 5 door sedan (rated for 5 adults). The model S has around 3 times the cargo space (30 ft^3 s "8 to 11" ft^2 on the BMW).
A 5 series is much closer, 5 adults, 18 ft^3 cargo space, and even leads by a bit on passenger space (98 ft^3 vs 94 ft^3). The model S is AWD only, so to be fair lets add that to the BMW 5 series.
The BMW 550i xdrive "base curb weight" is 4,372, the Tesla S P100D is 4883, so 11.6%. Both cars have slower/lighter versions of course. The M550i has a 0-60 of 3.8 seconds, the Tesla 2.5 seconds.
They are both handle 5 passenger, have 4 doors (ignoring the heavier hatchback on the S), have AWD, are within 1 inch wheelbase and 0.6 inch in length. Having sat in both I'd say that feel relatively similarly sized, especially in the back seat (unlike the M4).
As for the m2 competition, I think the weight distribution can be partially attributed to it being a small 2 door with a token rear seat and tiny trunk than any strong statement about ICE vs electric. I'm not saying batteries (at least today) are not heavier, just that for comparably sized/marketed cars the difference is relatively low.
I think a common trap is to compare a large tesla that fits a family of 4+dog and to compare it to a high performance sports coup... because of the similar performance. Not because they are comparable cars for most uses.
All your points are valid, with one caveat. The Model 3 Performance really is targeted at people that want the "fun" car, so I think it's appropriate to compare it to other sports cars. But that said - there is obviously a huge benefit going towards the Tesla in having more utility due to its size.
