SCP 835 scale

Just finished reading 835... wow!

But there is one thing I don't get: It was supposed to "only" weigh 4 tons. But the ones swallowed moved at a speed of 1 meter / minute and they spent a lot of time in there. It seemed to consist of pretty solid/ thick walls. I kinda fail to imagine the appearance of 835. How big / long is this thing and how does it manage to still be that light?