Measurement of ingredients for baking 400g of brown bread
2T instant yeast is ~17g, 2T of sugar and oil are both around 25g, 1t salt ~6g. ½t bread improver is going to be a gram or two.

So you've added together 300g flour + 100g water + ≈75g other stuff. By a pretty fundamental law of physics, mixed together that weighs 475g. Since you got 600g instead, there are two possibilities:


You've measured wrong (e.g., you added an ingredient twice, your scale is broken, you made a mistake when taring the scale, you accidentally included the bread pan weight, etc.)
You've got a Nobel prize coming.


As exciting as ② would be, I'm afraid ① is far more likely.

The bread rising probably has some effect on the weight—the yeast is performing a number of chemical reactions, using oxygen from the air and carbohydrates, water, etc. from the dough, and releasing CO₂—I'd guess it loses a little weight here, but I'm not sure—either way, pretty sure it's not a significant change.

When you bake the dough, of course, some of the dough (mainly the water) is going to evaporate. Not entirely sure how much. But of course you can find out by weighing the dough before it goes into the oven (remember to subtract the bread pan weight) and then after it comes out and cools. Then you can use that to scale your recipe to get reasonably close to 400g.

Personally, I'd make my first stab at it with a dough weighing about 450g. Scale down everything proportionally—and if you convert everything to weight, and use an accurate scale, that's much easier than taking 6% out of 2T (which is about ⅜t).