The Gemara says that when there is no קרבן פסח then מרור is only מדרבנן. There is a fundamental dispute between Tosafos and the Rambam what this means.
Tosafos understands that מרור is an independent mitzva that is only נוהג when there is a קרבן פסח. The Rambam however, states both in the Sefer Hamitzvot and in the Mishna Torah that מרור is not an independent mitzva but rather a part of the mitzva of קרבן פסח. One נפקא מינה is someone בזמן הבית who did not bring a קרבן פסח (because he was טמא or בדרך רחוקה). Does he have a chiyuv min hatorah to eat מרור? According to Tosafos yes according to the Rambam no.
The Rosh in ערבי פסחים is מסתפק whether you need to eat a כזית of מרור. He says you do based on the fact that you make a beracha על אכילת מרור and אכילה is never less then a כזית. The שאגת אריה is bothered by a number of questions. The Mishna on 39a states explicitly that you need to eat a כזית of מרור, also why not make a beracha of על מצות and eat less then a כזית?
RYBS answered as follows. The Rosh agrees that when מרור is דאורייתא you have to eat a כזית. His whole question was nowadays that it is d'rabbanan. RYBS explained the safek as follows. What was the takana of מרור? The reason for the takana was clearly זכר למקדש. There are 2 possibilities:
1. The takana was also זכר למקדש
2. While the reason was זכר למקדש the takana was to fulfill the mitzva of מרור m'drabbanan.
If the takana was זכר למקדש, then it would make sense that you would not be chayav to eat a כזית and there would be no beracha. Therefore the Rosh says from the fact that we make a beracha on the mitzva of מרור shows that takana was to fulfill the mitzva of מרור m'drabbanan and therefore it requires a כזית.