[export quality]

Thank you very much for your help.  Now I understand that the size control number of px, and the quality control the intensity of the signal at each point.  Is this correct?  Currently, I keep quality at 80%, then use the maximum pixel size allowed to make file size under 500k (some websites only accept files under 500k).  Or for a 4k monitor (4096 X 2160 px), Should I keep the shorter side of the picture at 2160 px, and use the maximum quality allowed to make file size under 500k?  Or for each picture, I have to try different combinations to find the best balance between pixel size and quality?

[export quality]

I am new to Affinity.  I want to export my picture to jpeg file, and I want to keep the file size as less than 500k with the highest picture quality.  

I have two options:

one is at size 1800px by 1200 px, quality 79%;

the other is 1200 px by 800 px, with quality 91%.  They yield files with similar sizes: 490k and 494k.

When they are viewed on the screen after resizing to the same size, the 1800 px by 1200 px one likes better.  Can you tell me why?  Is this always the case.  What is the best combination of size and quality under the file size constraint?  I have attached both files.  Thank you for your help in advance.