Orbital size of Carbon and Oxygen in Methanal I was given an introductory question into Carbonyl chemistry today and I wanted to ask a question about Methanal. 

The following picture was shown below of Methanal:



and I noted that the orbital sizes differed for both Carbon and Oxygen in the top and bottom diagrams. I thought one of these diagrams must be wrong.

I then knew that as Oxygen has more protons than Carbon, then its atomic radius is smaller, meaning the orbital size must be smaller too, so I thought that the bottom diagram represented the size of the p orbitals better both for anti bonding and bonding. 

Is the diagram for $\pi$ bonding drawn correctly in terms of orbital size?
The orbital size in the picture shows the coefficient in the linear combination of atomic orbitals, i.e the pi bonding MO has a larger contribution from the oxygen atomic orbital, and the pi anti-bonding MO has a larger contribution from the carbon atomic orbital. 

Here is another picture for a different molecule where they use the same convention, but also show the coefficients explicitly (source):




  Is the diagram for π bonding drawn correctly in terms of orbital size?


An orbital is not a "thing" so it does not have a size. It is a 3-dimensional function. The picture just shows the symmetry of that function and the node (where it is zero). For a slightly more realistic depiction, you can check out molcalc.