I went from 9st 2 to 8st 7 to 13st. I started with horrendous nausea that killed my appetite and could barely put anything in my mouth without gagging until about 18 weeks. Meanwhile I started buying maternity clothes at 8 weeks to accommodate my growing bump and breasts despite the weight loss.
I was too exhausted to move in the first trimester and by the time that wore off, SPD set in. I'm normally quite active but by 6 months pg my movement was getting quite restricted. By 7 months sustained exertion such as getting round the supermarket was too much and my last month I couldn't get out of the house unless someone else drove me door to door due to my immobility, not fitting behind the steering wheel and being 8n effect snowed in. I also developed carpal tunnel syndrome from the water retention which destroyed my sleep so I was waking hourly with a dead arm/pins and needles then having to painfully heave my pain ridden body over. I was too exhausted to read. I had no sensation for things like crafts. All I could do to pass a very long, lonley, painful month (with no effective pain relief) was sit at the computer playing computer games. By the last week I didn't have the strength to get off the sofa on my own. There was some comfort eating as a little highlight in the day, and an attempt to feel some kind of energy level.
I had a large amount of fluid in a huge bump and a big-for-me baby. Not huge, long and lean but over 2lb bigger than my petite birth weight. At 6 months I ended up having arguments with idiot strangers about my due date and it not being twins as I looked full term... yet still had my waist from behind. Until the last couple of months it was mainly bump and little fat gain on me (other than the breasts which grew 5 cup sizes by the end)
2 stones had gone by a couple of weeks after the birth despite the birth and complications causing additional water retention. The additional 2 stones came off gently over the next 9 months and I ended up lower than my original weight. No "dieting", just returning to my normal balanced diet, breastfeeding and rebuilding my fitness and lost muscle.
The next pregnancy was better. I knew that it wasn't "pregnancy aches and pains" as my GP called it, got some crutches bypassing the NHS's pointlessly slow protocol and kept up some kind of movement keeping off a stone of water retention. Baby only marginally smaller, but less fluid. Being a mere 3 stones above usual was a bit easier and kept the carpal tunnel at bay. Still the same "me" weight after and lost it again in the same way. 18 months later I ran my first half marathon.
My body just relatively gains a lot in pregnancy, partly because I'm small to begin with, then the immobility of SPD lowers my metabolism and wastes my muscles.