Let us begin with what to do when asked to lend eggs and proceed to what to do after the fact.
The mishna[1] states that one may not lend articles in a way that obligates the borrower to return articles of the same type and amount. This rabbinic form of ribbit, known as se’ah b’se’ah, is forbidden out of concern that the article’s price will increase and the borrower will have to return more value than he received. The problem, which begins at the time of the loan, exists even if the article’s price did not end up changing. Thus, it is not clear that one can lend eggs and receive eggs even of the same size.
Several permitted methods and cases of leniency are found in the gemara and poskim, but some are too complex to present in this forum. One example, which works well with things such as eggs (at least in Israel), is the concept of yatza hasha’ar.[2] It permits the loan of a commodity with a stable price, which either is set by the government or is otherwise fixed for at least days at a time (e.g., if there is a manufacturer’s recommended price, even if some retailers give reductions).[3] This heter applies only if the commodity is readily available[4] and if the lender can repay the loan as early as he wants.[5] In such a case, it does not matter if the price goes up before payment is made.
The most practical heter applies to cases of healthy, neighborly relationships. The gemara[6] rules that one may lend loaves of bread to a friend without stipulations. The Rama[7] rules like those who explain that possible small changes in the value of small quantities of a product are not considered purposeful interest. Rather, it merely represents a loose manner of accounting between friends. Although it seems that the Shulchan Aruch rejects this position,[8] causing problems for Sephardic Jews, it appears from the Beit Yosef[9] that if a set of neighbors has a clear, preferably long-standing policy of not caring exactly how much is returned, then even Sephardim may be lenient.
Both of these heterim are problematic when the borrower returns a clearly larger quantity than he borrowed.[10] However, if the owner states honestly when he “lends” the commodity that, from his perspective, he is giving a present and not a loan, then it is irrelevant if he receives more.[11]
Even if you should not have taken the larger eggs (which we cannot determine from your question), it was a case of no more than rabbinic-level ribbit because the additional payment was voluntary. Such ribbit need not be refunded.[12]