300MB is what you should expect for 9 images exported as uncompressed 16bpc tiffs (or psd).
Why the increase in file size? There are three things.
1) A raw file is only a third of a final image. Due to the color filtering
on camera sensors, the total amount of raw data is only the equivalent of a single channel. The rest of the data needs to be interpolated (an educated guess) to fill in the gaps for each red green and blue channel.
2) The kind of compression used has an effect.
3) The raw data is either 12bpc or 14bpc, usually 12 if it's a 35mm SLR. Tiffs and PSDs can only be either 8bpc or 16bpc so that 12/14bpc data gets placed in that 16bpc file and the gaps in the data are filled. Each pixel in each channel goes from having 12 bits describing it to 16. The final result is a file with more ones and zeroes describing your data and thus a larger file size.
If you want to reduce the hard drive space used there are a couple things you can do but they come at a cost. The first is to export at 8bpc but then you loose data and flexibility in editing. The second thing is to use compressed tiffs. this will nock your files from around 36mb each to just over 20mb each. However, there is a noticeable performance loss when saving and opening.