User blog comment:SonikFan112/New Reviews/@comment-2205861-20100905082515

Gotenks was already weak because after Trunks and Goten battled, they were injured and those two injured Saiyans fused and Gotenks was also injured. They also got many hits from Vegeta, so the Galick Gun won against Big Tree Kamehameha. Plus, they didn't fuse for 23 years. First fusion after Fusion Saga (DBZ) so they were quite rusty. Vegeta SSJ4 was obviously stronger than an untrained, injured Gotenks SSJ3.

P.S: I didn't make that part...