It seems that the prize pool was relatively stable for the whole time. Apparently, the web site where I get the data was just reporting it incorrectly for some reason.

On the general question, I'm not sure what factors contribute to the size of the prize pool. I suppose the price feed from the witnesses might be one of the factors, but I've never noticed a connection.

If we get to where SBDs are printing, I think that the price would almost certainly impact the number of SBDs in the pool.