Not exactly 'large' companies only. But if all those companies have worker representation and they raise wages, that would raise the equilibrium wages of those companies that don't have it.
Oh, then we have different scales in mind. A lot of the German industry is comprised of smaller companies with 50 - 2000 employees (the so-called Mittelstand).
These small - medium companies are the ones that have been creating jobs in the last decades, the larger companies tend to have less employees in Germany than they had 20 or 30 years ago (and instead more employees in other countries).
The Mittelstand is regulated less by employee representation but more by the fact that these are usually family companies and the owners live in the same (often small) town where the company is located. They have a reputation to lose and treating your employees badly or paying low isn't the best way to improve your standing in your hometown.
Of course, two generations later, when there's often less connection between the owner familiy, the company and the community, there will be different forces at play and that's where board representation of employees becomes important (and the German tax law, which gives the heirs rather big tax discounts if the amount of employees stays at least the same for a minimum number of years).