There are BIG advantages to working at a smaller firm earlier in your career, particularly in service industries.
1) you probably don't have a lot of people above you: you're probably not doing crap work like photocoppying, you might even have some responsibility.
2) you get to see everything: if you work at Citi or GS or Bain or BCG you might be assigned to an industry specific team, this can result in being 4 years out of college knowing only one specific industry. Can be great if you're into specialization, not so great if you're looking to see some different things and broaden your perspective.
3) some smaller firms have very interesting compensation packages.
As far as EC's go I would say that you should find something you really like (sports, community service, alumni associations, etc.) and get as involved in it as you can. if you like sports captain a team, help run a league, coach youth teams, if you like community service get on the board of an organization, lead an initiative at work, take a week off to go to new orleans and build houses, if you can get other people to go with you even better.
The point of all this is that when you submit your essay for somewhere you can back up your claims with EC's that go along with what you're saying. For example my Tuck essay said I'd contribute to the community/culture because of my background playing hockey and being very involved with it, my experience coaching and officiating and my passion for the sport. If I'd said that and didnt' have extensive EC's for hockey listed the discontinuity would probably hurt my application, but instead it shows a continuing commitment to a passion.