The base font size scales with the size of your smallest 1x1 tile. So in your case, it looks like number of columns on the screen has been increased to fit more tiles which shrinks the size of the smallest 1x1 tile (an approach often used to get ‘half-height’ tiles). So this causes the base font size to be smaller on certain tiles like the calendars and news feeds.
Feel free to skip the ‘more details’ if you’re just interested in an approach to adjust the size. 
More Details (tap to expand)
Here’s a comparison of the same ‘Normal’ sized tile on the left in a Reflowing dashboard (from before this release) compared to a ‘Scalable’ dashboard set with the same number of equivalent columns on the right. There’s some minor scaling differences, but overall they’re the same.
The difference is that on the right, when the dashboard is viewed on a bigger screen, the content of the calendar tile will scale accordingly and the relative size will appear the same.
I’ve made note of the feedback and I’m thinking we could add a content size adjustment option to the calendar and news feed tiles similar to what’s available on Label Tiles and Hero Attribute Tiles. This would help accommodate preferences as to how big you’d like the content to be and covers configurations like this where more columns are displayed resulting in a smaller 1x1 tile.
In the meantime, you might consider the Custom CSS approach @James noted in the following comment: