Why is the maximum recommended solvent volume for Findenser 1 L in a 2 L flask?

If you have too much solvent / the flask is over-filled, more vapour could be generated than the Findenser is able to condense effectively and so the Findenser may not contain all the solvent.  It is general good laboratory practice to fill flasks to a maximum of half full with solvent.

1 L solvent in a 2 L flask is the maximum volume our R&D Laboratory have tested, and so the maximum we can guarantee will work well.

If you use the Findenser carefully (e.g. do not overheat the solvent) and have a relatively undemanding application (e.g. a high boiling point solvent, which is therefore easy to condense), then you may well be able to successfully use Findenser with a slightly larger solvent/flask volume, although we cannot make any guarantees.