The image attached is from a paper few years back, it gives a good over view of what's going on. The key steps for the solubility of the molecule are as follows:
(1) The cleavage of the long aliphatic group to form chlorophyllide. By removing this large hydrophobic group you increase solubility by...
Ok, going to have to go back to first principles here. When you solvate a molecule you pack water around it in an ordered fashion. Water doesn't like being ordered as is represents a decrease in entropy (chaos, essentially the random nature of a system), and this takes energy to occur. The reason so many things are soluble in water is because water is a polar molecule (it has slight possive and negative charges on it) which means it can interact with charged substances (eg the +ive Na and -ive Cl in salt) and also with other polar molecules, where highly electronegative elements such as O or N have drawn -ve charge towards themselves (eg the OH groups on sugar). In the later case these interactions are known as Hydrogen bonds, as the slightly +ive Hs on the water can interact. All these interactions are energetically favourable therefore can counteract the un-energetically favourable decrease in entropy. Bit hard to explain without diagrams so if anyone cares i'll sketch some out.
Ok, so back to the cleavage of the aliphatic group (by the by, aliphatic referes to linear hydrocarbon chains, as opposed to aromatic groups, which are rings, like those found on THC
). With a bit of luck you'll now see how removing a large group with no charged or polar nature is good for the solubility of the whole.
(2) Next up is the removal of the chelated Mg2+ ion from the centre of the tetrapyrrol (tetra = 4, pyrrol = this particular type of 5 carbon ring) structure. Usually we like metal ions in terms of solubility, they have a nice strong +ive charge for the negative ends of H2O to cluster around. However, in this case the Mg2+ is tightly held in the centre of the ring so not too much water can get to it, and it is also hogging 4 Nitrogen atoms which are extremely good H bond acceptors. The Mg is thus removed, and water can get into the gap, resulting in lots more lovely energetically favourable bonding.
(3) Finally you get to the breaking open of the tetrapyrol structure. This opens up 3 more hydrogen bonding sites finally over-coming the entropy energy barrier and alowing the break down product to drift off out of your bud....