How to keep slide formatting when copying between ppt presentations?

HI there,

I have been making up new powerpoint presentations at work recently and to do so we use lots of material from old presentations and in some cases can simply copy/paste a bunch of old slides into a new presentation. We don't want to change those slides at all, they are fine as they are.

So far so simple, but we can't seem to prevent it changing the formatting when we do this which means each slide needs to be tidied up.

Generally it tends to greatly expand the text size meaning it all has to be reduced and moved around so it fits again as it did prior to moving the slide. This is confusing as the basic formatting we use for our slides in different presentations is quite consistent across all our presentations so we don't know why it's doing this.

I've tried to see if there is any easy fix to this problem - no doubt there is - but can't work it out!

Any help gratefully appreciated thanks!

David