Learn Swedish: Ett större antal studenter betalade en mindre avgift. - A larger number of students paid a minor fee.

This is called the absolute comparative. In Swedish, the comparative form is often used to express a moderate or significant degree of something without making a direct comparison. Here, ett större antal means a fairly large or significant number, rather than strictly larger than another specific group.

Does en mindre avgift mean the fee was smaller than it used to be?

Not necessarily. Just like större, mindre is used here as an absolute comparative. It simply means a minor or rather small fee. Swedish uses this structure to soften the meaning, showing that the fee is modest but not necessarily being compared to another specific fee.

Why is there no word for 'of' between antal (number) and studenter (students)?

In Swedish, you do not use a preposition like av (of) after words expressing quantity, measure, or numbers. You simply place the noun directly after the quantity word, similar to how you would say en kopp kaffe (a cup of coffee).

Why does the sentence use ett before större but en before mindre?