Early effort estimation is both essential and challenging in software development projects. Accurate effort estimation mainly relies on accurate software sizing. COSMIC Function Point (CFP) is a means for measuring the functional size of software. It is a standardized method that can be applied to obtain size measurements based on early artifacts such as the software architecture design. In this paper, we present an industrial case study conducted to evaluate the correlation between CFP measurements performed at the architecture design phase and the actual development efforts. The study is performed in the context of a FinTech company that develops a system comprising a backend, a native iOS application, and a native Android application. We collect CFP calculations that are performed at the architecture design phase of the corresponding project. We also collect measurements regarding the actual effort for 12 months. Then, we investigate the correlation between CFP calculations and the measured effort. The actual effort can vary for some development tasks that are associated with the same CFP calculations. We analyze the relation of these variations with the level of developer experience and the type of target platform. Our results show that there is a significant correlation between size estimations made with CFP and the actual effort. The variance in effort per CFP among varying types of platforms is statistically significant. We also observe variance in effort per CFP among developers of varying expertise levels.