I wouldn't say "big" tech company so much as "name" tech company. Lots of big tech companies exist (e.g. HP, EMC, IBM, GE) that generally don't carry any kind of magic imprimatur.

I think the previous response is correct in that there is definitely a halo effect that makes one seem more credible. Example was an article I was just reading entitled something like "Ex-Googler Gives Life Lessons" and it turned out the person was 25.