The common opinion is that the larger firms offer more experience. This is more or less true in the sense that they have a bigger client base and hence have more diversity in the industries and work you can experience.

However, that being said, whether you benefit from the size of the organisation also depends on your role and the attitude of your superiors.

Larger firms have the opportunity to afford you a more diverse experience and add a little shine to your CV later on when you leave. This is not to say that medium firms are no good. It all boils down to perception at the end of the day.

Taking the job now won't stop you from changing jobs in future if a position opens up in a bigger firm if that's your plan. Provided no contract/bond.