Increasing the decimal precision abruptly increased the size of table
Keep in mind that the dropped column is still physically present in the row.

But the real killer is probably that, if this is a heap (table w/o clustered index), then row updates that increase the row size may cause an avalanche of forwarding. Consider that if you start with pages at nearly 100% full then any row increase will cause the row to no longer fit in the page, so a forward record has to be left in place and the row has to be placed somewhere else. Repeat this for every row (extreme case) and you have a pretty nasty scenario. 

If is a B-Tree then the same scenario will cause page splits which are better because during the split the copied rows reclaim the space between the rows on the page, which the heap forwarding case does not.