Limited resources will inevitably lead to trade-offs between traits. However, trade-offs are not always observed, for a variety of reasons. For example, failure to take into account covariates, and high variation in resource acquisition relative to variation in resource allocation, can obscure the underlying trade-off. In the present paper, the trade-off between offspring size and number was examined in the common lizard Lacerta vivipara. Size–number trade-offs can arise for two different reasons, limited food resources or space constraints. There was no significant trade-off between size and number of young under natural conditions, whereas females captured before ovulation and provided with excess food under laboratory conditions showed a negative correlation between clutch size and offspring size. Food supplementation did not significantly increase reproductive output compared to natural conditions and, thus, the presence of a trade-off under ad libitum conditions was not the result of space limitation in the female reproductive tract. The variance ratio between allocation to investment, however, was twice as high for females ovulating in the laboratory, suggesting that the existence of a trade-off under ad libitum conditions could be a result of less variation among females in resource allocation.