reactivity and stability of alkene

Highly substituted is not more stable as both less and more substituted alkenes can have resonance effect and more substituted alkenes are only more stable than alkenes with shorter chain. In fact, they are more reactive as the R groups can have more +ve inductive effect to make the C=C bond more e- rich. So there is no contradiction.
(p.s. Resonance effect depends on how long the carbon chain is, but more substituted doesn't necessarily mean the chain is longer.)