Arguing about lines is useless. With good application design you can greatly reduce the number of written lines (even in terms of magnitudes). For some parts you can also try a new, different, approach, reducing the number of the lines even further.
A simple linux application 'tree' has around 90 KiB of code (around 3.5k lines I think, but thats just a guess), written in C. You can do the same application in Haskell on less than 400 lines, my version (simplified) has around 12 KiB (with tons of comments) ... there are even shorter. So yeah, by changing approach to the problem, you can reduce code written (and if you know what you're doing, you can also reduce time spent coding/thinking how to code it).
It is the same with good application design, if you cleverly separate your program to parts (even making some parts of it modular), you can save a ton of time ... although this means that you need to think forward.
First of all, why do the AAA games have so many people working on them? The answer is simple, most of it is art. Creating good art takes really a lot of time, and for large open worlds you need a TON of art, and what is more important, it needs to look consistent in different parts of world (so basically small team of people is better than huge team here). The less artists in the team, the higher art consistency is. I don't know about 2 painters producing exactly same mood with the picture - this applies for games too. Although less artists = more time to get art done.
Open world games are doable in small teams, especially if know how to use tools. Generating terrain and nature is not really hard, filling the world with animals and enemies can be automated, but filling the world with story and interesting stuff can't (just not yet, procedural quests are still the worst one out there). This is where you will need the help of others, or your world will either look empty, or it will eat huge amount of your time.