MBP13: 3.02 pounds (1.37 kg) MBP15: 4.02 pounds (1.83 kg) XPS15: 4.4 pounds (or 3.9 pounds with smaller battery)
MBP15: 15.5mm 349.3mm 240.7mm XPS15: 17mm 357mm 235mm
But I agree - if you're wanting maximum portability, go with the smaller 13" device, which is 33% lighter.
But then you may want to compare the XPS13, which is smaller, lighter, newer CPU, has a larger battery (dell mentions 18h, reviewers tested 10h+, but actual battery life comparisons TBD), and is also limited to 16gb.
Yes, different compromises.