I've never noticed an issue here. Looking into it the thread messages default to 1rem, your browser's default font size. So if this isn't suitable for you you may want to change your browser's font size.
The font size in the feed is 1.25
x your default font size, which seems fairly reasonable for titles. But if you want to change that you can apply some user styles. For example to make it only 1.1x as large:
h5, .h5 {
font-size: 1.1rem !important;
}