The reason why humans are growing bigger (excluding any fat jokes) is because bigger humans must eat better, thus are more capable of surviving, and are also much better fighters (better strength/reach). This led to size being considered a favorable trait, leading humans to select larger mates and thus evolve to be larger. Once we started dominating the environment, we all grew larger, but the same instinct persists even today (just ask any guy under 6 feet).
As for the dinosaurs, there was much more oxygen and CO2 in the atmosphere, so plants had more soil to grow in, the earth was hotter so more precipitation occurred, causing massive rainforests, and most importantly there was lots of O2/CO2 to breathe (plants breathe CO2, which gives them the majority of their mass).
Over millions of years this C/O was sequestered underground, and things evolved to be smaller to compete as the earth starved of life-giving Oxygen and Carbon. Now we are putting it back in circulation, reversing this trend. However, this reversal causes changes, and as change is scary it is by default bad.