From a vehicular scale POV it makes more sense. A
Lamborghini Aventador is about 4.8m long and 1.1m high. I don't know what the exact dimensions for a Western Star 5700 are (the WS5700 is nearly 5x the weight* of a Lambo Aventador), but I'd wager that they'd be significantly larger than a Lamborghini Aventador. Sure, Lockdown is almost the same height as Optimus Prime in the movie, which appears to be a scale discrepancy. I guess AoE Lockdown can mass-shift between his car and robot modes. A Voyager Class Lockdown may be more to scale with Optimus Prime in robot mode, but he'd be oversized in vehicle mode.
Also, I guess that Hasbro didn't want to have a larger and more expensive Voyager Lockdown toy cross-competing w/ Voyager Galvatron.
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*Lamborghini Aventador is approx. 1.6t (1575kg); whereas the Western Star 5700 is approximately 7.7t (7711kg).