Not the same thing
QNAP does proper old fashioned RAID, striping across drives or mirroring, expansion involves rebuilding a drive from parity, basically an exact copy of the previous drive on a larger disk, repeat for all drives then enable the extra storage space. Performance is notably better if all the drives are of the same model.
Drobo does block level mirroring across whatever drives it decides are best suited for the data, if you drop a drive and replace it with another of different capacity, you'll end up with different blocks written to it than the original, essentially, what drobo does is ensure all data is on the array twice, it doesn't matter what type or size the drives are because they don't need to be in sync to read data off, drobo just reads from the most convenient drive.
That's the theory, and technically it works well, you can mix and match any old drives in there and expanding the array requires *nothing* more than popping a drive out and sliding a bigger one in, in practice however, throughput steadily slows down until it's unbearable to work with and starts causing timing issues with some applications, the only fix I've found is to clone the entire array, wipe it and then copy everything back, this will need repeating every x months, depending on the data and churn rate.
Fine for backup, no good for live storage.