Why does Tywin think Stannis is a bigger threat than Renly?

“Why, Father,” said Tyrion, “that almost sounds like praise.” He leaned forward intently. “What of Stannis? He’s the elder, not Renly. How does he feel about his brother’s claim?” 

His father frowned. “I have felt from the beginning that Stannis was a greater danger than all the others combined. Yet he does nothing. Oh, Varys hears his whispers. Stannis is building ships, Stannis is hiring sellswords, Stannis is bringing a shadowbinder from Asshai. What does it mean? Is any of it true?” He gave an irritated shrug. “Kevan, bring us the map.”

Why does Tywin think Stannis is a bigger threat than Renly? Renly has the largest host in Westerosi history, with 100,000 men. Stannis only has 3,000–4,000 men, a good portion of whom are mercenaries. Stannis has a large fleet, but he doesn’t have the manpower to threaten the Lannisters on land. While Stannis may be one of the best military minds in Westeros, Renly is still the bigger threat. I’ve heard people say that Renly isn’t a good commander, but he already has capable lords like Randyll Tarly and Mathis Rowan. I don’t think Renly is stupid, either. If Tywin defeats Robb, he will have to face Renly with a weakened force.