Then I compiled the same code over with 1.6.4 and the reported size is now 46%. And it doesn't even work! But that could be an issue with OneWire.h that I'll post separately.
Fortunately the Downloads folder of my browser still had the 1.6.1 zip and I could restore my sensor.
Has anybody else noticed the compiled code size difference of 1.6.1 (at May 5th, 2015) vs 1.6.4 (at May 17th, 2015) ? My fault, I haven't checked the actual bytes value, only the reported percentage.