There is one surefire way to make your client's logo bigger without actually making their logo bigger, but it is reserved for only the most desperate situations. You must have exhausted all other possibilities. Moreover, you must have run out of any patience, respect for your client, and scruples. Here's how it works.
You present the work with the too-small logo, and the client explains that its size must be increased. Don't argue. Instead, listen very carefully, nodding, drawing out detail and nuance. Make it clear that this is a matter of importance and complexity, and the client is right to focus on it. Finally, announce, as if it's just then occurring to you, that there is only one way to get this exactly right, to make sure that the client is absolutely pleased. You will prepare not one, but five options, changing the size of the logo on each one just ever so slightly. In this way, and in this way alone, can a reliable decision be made. Take my word for it: no client will turn down this offer, since the one thing these kind of people like more than arguing about logo sizes is looking at lots of options. Take the work away and promise to return to the next meeting with this exercise ready for review.
Prepare a new presentation with five different logo sizes: big, slightly bigger, slightly bigger again, slightly bigger still again, and biggest. Ideally, the difference between in the sizes should be maddeningly imperceptible. At the next meeting, and with a bit of ceremony, lay them out before the client left to right, smallest to biggest. To enable a robust discussion, the options should be labeled 1 through 5. As you spread them out, say, "We started with the original and changed the size in increments of [insert some small unit of measurement here] so we could get a really good choice." Let the client examine this array. This is a period of intense deliberation, partly because the difference in the sizes is so hard to distinguish. Into the silence that inevitably ensues, with some hesitancy, offer your point of view. "After looking at them all together for a while, we decided that number 5 was really a bit too big, and we were torn between 3 and 4." If the client is polite, they will pretend to care about your opinion for a moment. If not, they won't waste time. Either way, they'll announce that number 5 is the best and that's the one that should be used. Just for the hell of it, at this point you might want to try to make a (no doubt futile) case for number 4. But it doesn't matter: you won.
You may have already guessed how this works.
When you put together the presentation, the logo from the previous meetingthe size you originally showedis number 5, not number 1, and the options are all incrementally smaller, not incrementally bigger. But notice that you never claim otherwise. You say only that you've "changed the size ever so slightly"true. You say, as you lay them out, "Here's number one...two's a little bigger...a little bigger again..." etc.true again! Even your (admittedly cruel and perverse) advocacy for option 4 is based on an unspoken but nonetheless legitimate realization that the logo was actually too big in the first place, not too small. If you are very careful, you never have to lie at all.
Like all con games, this one is based on the illusion that the sucker has the advantage. In this case, it's the conviction that this kind of client always has that it's your job to do as they say. Little do they realize that your final allegiance is not to them, but to the quality of the work, something that you cannot in good conscience permit them to jeopardize with their lack of taste.
Two last things. The key thing is to make sure that no copies of the original presentation are left behind, lest they be produced for comparison's sake at the second presentation. You can usually secure the original by explaining that you need them to create the painstaking variations, and moreover that you can't bear the idea of flawed design work remaining in circulation. Finally, if the worst happens and you actually get caught, just blame the whole thing on a misunderstanding by an intern the client has never met that you will fire as soon as you return to your office.
For the record, I have never actually done this.