I had stayed at each Rembrandt building once over the last 4 months. The first one was in the newer building and even the basic room was spacious. The second, I didn't know there was a second building at the time and I found out that I booked the basic room in the older building. I was upgraded without asking to a suite, which was massive - so I never actually saw a basic room in the older building. This time, I booked a basic room again, presuming that it was probably a reasonable size since the other rooms were. That was a definitely not the case. It was shockingly tiny, the two single beds hardly fit in - so out of character from my previous experiences. In the hotel's defence, they had initially offered an upgraded room again without us asking but their suites only have one king bed. As luck would have it, this is the one time that I needed a two beds since a friend - definitely NOT a romantic relationship - came to join me in Bangkok, so we had to decline the upgrade. I'm sure they could have "upgraded" us to even a basic room in the newer building but that didn't happen.
The room is so small that even the slightly lower price they charge for it isn't justified, imho. The hotel could get away with its age in the suite but it was impossible to mask in this tiny room no matter how clean it may have been.