I'm having a few problems with my webserver, so I just want to verify this : Does the size of a big mysql database have any effect on the overal performance of a server ?

Currently my db is 1.4 gig big and my server is really working slow with a normal number of visitors. Only at quiet moments is everything loading fast. A few weeks ago it was only 0.9 gig and there were no problems at all, even with high number of visitors everything loaded well enough. Is this co´ncidence or not ?

Also : if size does matter here , are the 'active' databases the ones that determine the limit, or all mysql databases that are located on the server ? I have some backup db's there too, also 1.4 gig big each... and if you count them all together, it becomes quiet a bit.

(server : intel dual xeon 2.4 GHz, 1 gig ram, debian)