I love the scale charts. They're the primary reference I go to when determining how figures should scale next to each other. However, I think there are limits to how much precision you can glean from them. Like Sabrblade points out, they aren't all done in a similar pose, so you can't always get a good 1:1 comparison. There are also issues in that there are multiple official scale charts that aren't perfectly consistent with each other. For example, while maybe not completely relevant for SS86, the Headmasters scale chart on tfwiki shows Galvatron just a tiny bit taller than Rodimus, basically from his head crest.
So while they're the primary reference, what I take from them is that Optimus, Rodimus, Megatron, and Galvatron are all supposed to be about the same size. Add the simple fact that Galvatron is supposed to be a more powerful Megatron as a secondary source, and my final conclusion is that Galvatron being notably smaller than Megatron just feels wrong. Again, there's some wiggle room, but this case exceeds any allowances I'm happy with.