As a former Bodybuilder, I can relate to the comment by your trainer. His comment reflects one of the theories that was prevalent years ago. As Bodybuilding and Weight training evolved, there have been many advances in training theory. Some of the theories that we thought were valid are no longer the norm.
With respect to getting more defined, I can tell you from anecdotal evidence that I found that maintaining the same number of reps and weight while tightening up my nutrition is what works for me. This seems to be borne out by a University of Alabama study, High Reps vs. Low Reps for Fat Loss:
“One study from the University of Alabama in Birmingham showed that
dieters who lifted heavy weights lost the same amount of weight as
dieters who did just cardio, but all the weight lost by the weight
lifters was fat while the cardio group lost muscle along with some
fat.3. The common belief is that high reps magically get rid of fat.
While high reps with light weight to fatigue can create a muscular
response, it does not necessarily remove fat better than low reps with
heavy weight. While more studies are needed to compare the fat loss
effects of high reps vs. low reps, substantial evidence is mounting
that it’s not necessarily the amount of weight that is used, or the
number of repetitions that helps burn the most fat, but the intensity
of the workout. The goal is to create muscular failure with less rest
between exercises, which can have powerful hormonal, metabolic, and
calorie burn effects (See: afterburn effect). In addition, for fat
loss, proper nutrition will have a MUCH greater impact on fat loss
than the specific rep range, or even workout.”
I, for one, would rather tinker with my diet and keep the same number of reps and weight. I feel this approach provides me with a better chance to retain muscle mass while restricting my diet.
As for “Bulking”, I’m not a fan of that practice since I see it as a form of yo-yo dieting. I’d rather gain muscle in a linear fashion without having to gain excess extra fat along the way. I do this by maintaining my reps and weights while supporting my body with sufficient nutrition and rest.