Not if you understand the importance of repetition. Generally, your new audience won't be any more inclined to buy after seeing your ad just once than the first audience was. So, by quitting the first magazine to advertise in the second, you'll probably just waste your money and receive a disappointing response from both ads. Because repetition is the heart of effective advertising, you should continue to advertise in the same magazine for several consecutive issues. To ensure that the same audience sees your same ad enough times to begin to trust you and therefore buy your product.
Size is secondary.
Most novice advertisers think that the larger their ad, the more sales they'll make. Of course, ad size is important since prospects tend to place more confidence in a large display ad than in a small classified. But ad size is of secondary importance when compared to ad repetitions. But one full page ad is not likely to generate as many sales as six, one-sixth page ads published in six consecutive issues of the same magazine. Ad repetitions are far more important than ad size.