An increase in sales

An increase in the number of sausages

An increase of 30%

An increase of 2,500 sausages.

I would use in with nouns and of with numbers and percentages.

Wow.  It's hard to describe.  Decrease IN will be correct about 95% of the time.

I hope this helps.  I'm not an English teacher; just a native who reads a lot.  (I wouldn't know a past participle if it jumped up and bit me on the ass!)