My general sense is that pretty much old everything is big. But what if fossils expanded with time?
Not really true. The number of different species that have lived on the earth, it would be astonishing if the largest species to ever have lived had evolved recently... but this is exactly what has happened.
Also, there are some things (I hesitate to call them species) like the Coelacanth which have stayed very similar in form over hundreds of millions of years.
Large animals get more press. I'd guess they also fossilise better and are easier to find.
Regarding large insects and arachnids.... no one is completely certain, apart from the fact that there was a much higher oxygen content in the atmosphere during the Cambrian, allowing larger sizes. This does not explain everything though (but what does?).