The first number denotes the magnification of the binocular so 8x42 means 8x magnification. That would mean you would see an object that is physically 80 feet away as though it were only 10 feet away.
The second number refers to the diameter in millimetres of the objective lens (the glass that is furthest from your eyes when looking through the binocular) e.g. 8x23 means a 23mm diameter lens. This determines how much light can enter the binocular and also the overall size of the binocular.
For Opticron, a compact binocular (or monocular) is classed as such if the objective lens diameter is less than 30mm. Objective lens diameters of more than 30mm are classed as 'full size'.