The shorthand answer is that the age of colonial empires was not a board game where each empire has a a strategy, and there's a way of scoring a winner. Empires -- like everything human -- were complicated with multiple (often conflicting) motivations and multiple (often conflicting) views on how international relations would evolve.
Doubtless some colonies were seized just to add acreage to the Empire, but there were so many other reasons:
- Raw materials (the spice islands, gold, etc.)
- Coaling stations to facilitate haulage between home and important sources of wealth
- Defensive: It's a great place to dominate a narrow sea (e.g., Gibraltar)
- Winnings: After a war (e.g., the Seven Years' War), the loser frequently had to pony up some colonial possessions to the victor.
- Ma, it followed me home, can I keep it? The British in particular seemed to keep picking up territory when some entrepreneur went off and conquered something for personal gain and added to the Empire
- Pure accident. Captain Whatsit sailed by, discovered an island, claimed it for Home, and as it was (a) distant and (b) worthless, no one contested it.
This is not how a tidy Empire with defensive borders is created! Why didn't they regularize the borders?
First, in each colony, there were people on the ground who owned property there. No matter how much geopolitical sense it made to trade a chunk of Existan for the Island of Whatsit, the rich colonials who owned chunks of Existan would object. "What? You mean to give up the part of our Sacred Empire? Don't you care? If you really want Whatsit, go conquer it like a civilized person."
Second, until fairly late in the colonial game, the borders were where they were because they didn't matter that much. The colonies were centered around rich territories of strategic locations, and the hinterlands were full of natives, if that. Bordering rivals were in much the same position. The small armies in the colonies could not seriously threaten the other Empire (which could always bring in bigger guns), and Empire vs. Empire conflict would be in Europe or on the high seas.
Third, many of the colonies existed for strategic reasons and needed to be right where they were.
Bottom line: Multiple reasons for having a colony plus multiple conflicting private interests equals a confused muddle.