The most extensive imperial expansion during the times you listed would be Africa, and you already stated why; technology.

Two main techological advancements helped with colonizing africa. Steam ships allowed lots of men and supplies to navigate the many rivers that served as the only water way access to many land locked nations such as the congo, before this the only way through would have been a treacherous and dangerous 3-6 month trek through the jungle. Another reason is guns, Central Africa was a dangerous place and if you had to take months to get to a location then chances are you would encounter dangerous animals and hostile natives. Although guns had been around for centuries, it was not until they progressed to more modern firearms in the 19th century that they were able to fend off hostile encounters.

Another reason is also what you mentioned; ambition, or rather , necessity. By the late 19th century the opportunity for colonies was getting smaller. To nations like Germany and Italy who had just emerged and desired colonies, Africa was one of the few places left where this could be acheived. With more and more nations looking to grab what was left, it created a frenzy to get there before the other guy does.

Im sure other people can add to this but as you said in the title, technology and ambition.