Hi krs18,
Because your second question was also one of mine;
.single-jetpack-portfolio #primary {
width: 100%;
}
That’s the additional .css code you have to implement into your child theme. (answer came from this website by the way)
As of your first question, I have an idea what you mean. I think it’s all about different screen sizes. My development project looks very different on an iPhone 7 then on my MacBook or iPhone SE… I don’t have a clue what to do about it though…