How did the Roman empire come to be so big?

History Extended Essay (up to 2000 words). The question is: how did the Roman empire come to be so big?
Spell-checked;
Grammar-checked;
Personally proofread;
Title is centered, in bold print
Minimum 1000, maximum 2000 words
Recommended structure:
Introduction (problem setting)
Critical analysis, key arguments pro and contra (if applicable for the topic),
examples proving these arguments
Your personal approach and attitude to the problem with proving points
Conclusion
clear structure (introduction, main
body, conclusion; linking phrases are
used in a logical way)
30
- the message/main idea is understood
and formulated clearly, the opinion is
expressed concisely (thesis statement,
clear argumentation, no factual or
logical errors, conclusion matches thesis
statement and arguments)
30
- vocabulary and grammar (no simplistic
vocabulary and grammar structures, no
rude grammar errors)
Providing different viewpoints is required (+ using quotes and lots of different historical sources)