You calories will go up, not down. It's a common misthinking about how it works.
At 100kg you can sustain quite a large daily deficit and lose a lot of weight each week. While your body definitely takes more calories to maintain than a 70kg body, the ability to create a bigger deficit means you can eat lower. Eg you might be able to eat 1300 calories per day to lose 2lb a week.
As you get closer to 70 you will not be able to sustain such rapid loss weekly and the amount you can lose goes down. So you would eat say around 1500 to lose a pound a week.
When you get to 70 you're not wanting to lose any more, so you would eat say around 1800 to maintain that weight.
As Heather said, if you save the Weight Goals page or the Fitness Goals page your food range is recalculated. Save those often and remember as you lose that you should aim for less per week, eg 2lb/week at obese, 1lb/week at overweight, half a pound/wk when the last few are left.
Deb, in New Zealand