The short answer is no.
I know many people, myself included, that shoot 80-100gr bullets in .243 and never have a problem and we consider these bullets "heavy enough"
That is true, but a 243 bullet at 100 grains has a sectional density of .242 on the other hand a 270 bullet at 100 grains has a sectional density of .186, the excellent 130 grain 270 bullet has a sectional density of .242. So in reality the 100 grain 243 bullet and the 130 grain 270 bullet should, theoretically have similar terminal performance (that is discounting momentum).
The accepted minimum sectional density for hunting medium game is .200 to .210.
Sectional density is a bullet's weight in pounds divided by the square of its diameter in inches, and is an fairly reliable predictor of terminal performance.
An excellent article on sectional density.
http://www.chuckhawks.com/sd_beginners.htm
The second and actually more important reason is bullet construction. Typically in light for caliber weight bullets, manufacturers design the bullet to expand very rapidly - simply the bullets are constructed to perform like a varmint bullet with explosive terminal performance. The medium to heavier weight for caliber bullets are constructed to provide a more controlled expansion and better penetration for larger animals. This is true of the 243, 270, 30-06 and others. The very heavy, for caliber, bullet tend to be constructed in an even tougher manner for the largest game, ie the 180 grain 30 caliber bullets are considered adequate for Elk.
So a 243 100 grain bullet is designed to perform well on medium sized game, but the 100 grain 270 bullet is designed as a varmint load.
But my first reaction is to consider a 100gr psp; the same GN and bullet I shoot in .243; in a bigger caliber IE. 270 to be to light, where is the logic in this.
The logic in this in an understanding of bullet construction an sectional density. Can a 100 grain 270 caliber work for deer? Yes, is it a good choice? No. Can if an individual choose a more toughly constructed bullet (Barnes X) and do better, yes and no. The Barnes will perform better than the average 100 grain 270 bullet, but it still lacks the sectional density to reliably give good penetration on oblique shots.