When a buyer's purchase represents asignificant portion of a supplier's revenue, it increases the buyer's leverage. The supplier has a vested interest in maintaining the relationship and is more likely to make concessions to preserve the account.
"Buyer power increases when the buyer represents a large portion of the supplier's total sales. This dependency increases the buyer's ability to influence terms." (L4M5 Commercial Negotiation, 2nd edition, Section 2.3 - Bargaining Power Analysis)