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.
If we knew the spherical half-lens equation (dioptria) we could calculate the size of air bubbles inside a glass or gelatine sphere (or in scent gelatine candles).
For example, a bubble inside a sphere of a diameter of 20 cm (say, 15 cm from the 'front' surface of the sphere) in which case R = -10 cm in the dioptric power equation, seems to be 1.6 time bigger, if it floats in wat (n = 1,33) and twice the original size, if it is submerged in glass (n = 1,5).
If we place the bubble closer, for example 5 cm from the front surface, the enlargement is reduced to 1.2 times. In the critical point on the other side of the diameter, the object is enlarged n/(n - 2) times, for glass it is 3 times, regardless of the radius.