Order of bond rotation about alkene
First, recall that a single bond has the highest free rotation possible (and hence it shows conformational isomerism). A double bond instead does not show any rotation at all (hence, $\ce{abC=Cfg}$ type molecules show geometrical isomerism).

So, it's obvious that, the more double bond character a bond will have, the less likely it is to rotate freely. Here, we're talking about double bond character, because, due to resonance effects, the resonance hybrid in these compounds will only have a partial double bond character.

Thus, you only need to compare the partial double bond character in each compound, and apply the above rule. Bulkiness of the groups isn't really a factor, because the groups actually aren't that bulky, and it isn't a t-butyl group anyway.