Varying sizes in LUMO of acrolein

I want to ask a question about the varying sizes in the LUMO of acrolein during a 1,2- or 1,4-addition. 


Today in our introductory lesson to Organic Synthesis with 1,2- and 1,4-addition we were listening to the reason why some nucleophiles may attack one functional group compared to another. 


The following picture was discussed:


[![enter image description here](https://i.stack.imgur.com/Fn29E.png)](https://i.stack.imgur.com/Fn29E.png)


Now, as shown here, the far orbital on the alkene is far larger than the orbital on the $\ce{C}$ of the $\ce{C=O} $ bond. This implies that for a larger orbital, a nucleophile finds it is easier to attack the alkene than the carbonyl. 


This makes sense. 


However, when finding an explanation for these varying sizes in orbitals, I couldn't find anything to explain this. 


This question was the closest which explained the effects of varying orbital sizes but did not differentiate between $\ce{C1}$ of the alkene and $\ce{C3}$ of the carbonyl. 


Why are there two different orbital sizes?


