The size is irrelevant, the energy released is significant either way and definitely beyond the thresholds of hydrophones. During the USCG press conference today it was stated that the implosion made a significant soundwave as a result of the implosion, and I think in the coming weeks we'll hear reports of findings in regards to hydrophones which picked up the noise.

It also probably was armed with torpedoes when it was sunk, which probably detonated due to the explosive decompression.