The oil volume factor may be responsible for this problem. There are greater pressuse and temperature in the reservoir than the surface, leading to the diissolution of gas into oil. However, on the surface, these oil will degas when the decrease of P and T. So, the volume of oil will decline.
Oil can be described as a fluid (slightly compressible fluid); hence for this reason, pressure and temperature affect its thermophysical properties.
In the petroleum engineering literature, the oil formation volume factor (Bo) or the oil density captures this change in properties with pressure and temperature.
Hence this is why oil shrinks when it comes to the surface.