Not only do those MP weigh more...
Pentaprism versus penta-mirror weighs more.
98% coverage, 0.95X magnification viewfinder weighs more than a 95%, 0.8X mag VF.
Metal chassis vs plastic weighs more.
Add'l shoulder LCD screen increases weight.
A 1 million pixel, 3", articulated, rear Touch screen weighs more than a fixed, 250K pixel 2.5" screen.
A shutter able to do 1/8000 weighs more than one that tops out at 1/4000.
Might add some weight when flash sync is 1/250, instead of 1/200.
A camera rated for 100,000 actuations is more robust than one rated for 50,000.
Sealing for weather resistance adds a bit of weight.
7 frames per second vs 3 fps continuous shooting weighs more.
ISO 100-12800 weighs more than ISO 100-1600.
19 AF points vs 9 AF points weighs more.
19 dual axis/cross type points vs 1 dual axis point weighs more.
Dual Pixel AF in Live View probably increases weight.
WiFi adds some weight, too.
HD video capabilities must increase weight, as well.
Add it all up and it's pretty surprising the 70D only weighs 200 grams (7 oz.) more than the Xti!
Not only do those MP weigh more... br br Pentapr... (