Songs take less space in Renoise 1.8 than they would have in 1.5 and before. That is because the samples inside the songs are being compressed. Renoise uses lossless compression: once decompressed, the original samples are restored without any loss of quality.
One of Renoise 1.8's best kept secrets may be the possibility to manually replace lossless samples with smaller, lossy ones. Lossy compression allows you to bring an 35 MB song back to 2 MB without degrading sound quality too much.