Bubble sphere

Why do the bubbles in the back of the sphere seem to be bigger than those in front? And the oval blue flower from Venice is also bigger than the red in front of the sphere?

The glass sphere enlarges objects which are inside. The water sphere does the same (or a glass) but in a bit smaller degree.

The enlargement of an object inside a sphere is determined by its position, a principle which has made a monster out of this dwarf.

http://dydaktyka.fizyka.umk.pl/soczewkigrubasy.html (Pl)