Actually, on paper the Imagawa would've been ran over by the Oda if not for its alliances with the Takeda, Hōjō, and Matsudaira.

You are perhaps looking at a political map and the physical area each clan controlled. Take a look at this map I made. One on one, the Oda had not an insignificant economic advantage over the Imagawa.

A few things kept the Oda in the fight even when after Nobuhide's defeat in 1548 at Azukizaka. First, Nobuhide was quick to realize he was on the ropes, and had Nobunaga marry the daughter of Saitō Dōsan so Nobuhide could secure a peace/alliance and concentrate against internal rebellions and the Imagawa.

Second, the lords of Mikawa seemed to have been quite fractured over which side to take, or were fairly independent. It seems this area needs more research, though sources are lacking, but the Imagawa either didn't or couldn't expand its control into Mikawa very fast. Some of the lords of Mikawa definately backed the Oda, despite what the Matsudaira wanted. Even on the eve of Okehazama, it seems the Imagawa did not control the entire Mikawa province, and it is extremely likely that support of the Mizuno clan at Okehazama, a Mikawa daimyō clan whose realm crossed the Mikawa-Owari border, was vital to the Oda victory.

The same could be said for the Oda's precarious hold on the Mikawa-Owari border, with the Yamaguchi switching sides after Nobuhide's death. But the big take-away here I think is that the Imagawa didn't have as large an advantage over the Oda as often assumed.