It's water retention and blood volume! Your blood volume doubles really early in pregnancy so there's enough to grow the placenta. That's why people feel dizzy and get really thirsty/pee more as one of their first symptoms. All that water and blood weighs a lot. X
I believe it is, or can be, everyone is different just like our bodies are all different; we'll all be carrying our babies differently. I remember with my first pregnancy I started gaining really quickly, and nothing different about this one now too...however I an quite short so baby needs to go somewhere, and I gain weight pretty easily anyways besides that. So I'm still exercising of course, walking every day and doing my best to eat right; even whilst still giving into all of my pregnancy cravings.