Maybe differing font sizes is a means of either filling a page, or squeezing the text to a size where it can e fill one or more pages fully, without resorting to a paragraph or two on another page. This would make more efficient use of the space available.
I have no particular problem with this. It probably goes towards minimising costs, by providing a little more page space per issue. (More articles per issue)
With regard to fonts, and colours, they should be such that clarity is maximised. For everyone, most especially the “more mature” among us, the contrast should maximise visibility.
As long as the font is clear, it is less important. My favourite is Arial, and Times New Roman is also good, as are the fonts used by London Transport and on Motorway signs..
We can do without fancy / unusual fonts, just for the sake of it.
If the text is not easily visible and read, there is no point in using it.
Epic example, some company had spent a lot of money on the font on the back and sides of their lorry; very trendy, but scarcely readable!
Not a route to follow.
Howard