I don't know if I would classify Dortmund as one of the biggest clubs. Footballwise, they are, but not clubwise. They are in the middle of the progression to become one, but it's still a very fragile club that could collapse in every minute. Once they do, then they wont have the financial strength to get up again. The past years have obviously brought in some lucrative sponsoring deals and a fair share of fans worldwide. I remember someone saying "Dortmund is probably the most supported 2nd favourite club in the world" but all that is not enough to keep up with other top clubs who get twice as much revenue even if they were domestically relegated.
Dortmund is a club of the size of Valencia, Tottenham, Napoli, Schalke, Porto etc who are just overachieving since some years. But it will take at least(!) another 5 years on that level till they become recognized as "one of the biggest clubs", and every season - every transfer window - might disrupt this momentum and stagnate their progress. I really hope that they can become a constant big club, and not just one inbetween the 2010s years.