I gotta say that I was pretty disappointed when I saw this guy's prototype pics and later, its release pics on the 'net. In robot mode, he is little taller than a standard Binaltech/Alternator. And what's wrong with that?
What's wrong is that he can't even stand shoulder to shoulder with MP-01 Optimus Prime. He is in fact almost three whole heads shorter than MP-01 Optimus Prime when he should be towering over the latter.
Three and bit heads shorter than MP-01 Optimus Prime
MP-08 Grimlock is simply not to scale. The Transformer Masterpieces are approximately 1:24 scale because MP-01 Optimus Prime is very close in scale to, if not exactly to scale with, a Binaltechs/Alternator and the latter series is supposed to be 1:24 scale.
The Transformers Masterpieces were doing well so far, even MP-05 Megatron is to scale with MP-01 Optimus Prime (because they stand shoulder to shoulder), that is until MP-08 Grimlock went and threw a spanner into the works... interestingly, not an act he is foreign to judging by his G1 disposition.
(Disclaimer: I know the seekers, MP-03 Starscream, MP-06 Skywarp and MP-07 Thundercracker, are not 1:24 as well, but they cannot have been because a 1:24 scale jet would mean that the seekers would have to be more than 5 times the size of MP-01 Optimus Prime, and that would just be impractical. So, they were done as best they could, which is roughly slightly shorter than MP-01 Optimus Prime, like in the G1 cartoons.)
MP-08 Grimlock looks a bit like a joke next to MP-01 Optimus Prime.
Left - Grimlock hulking over Optimus Prime in the G1 cartoon
Right - Optimus Prime towering over little Grimlock