[size=+1]GT2[/size] does not have the complete per-event records that [size=+1]GT1[/size] had. That's part of why the save is smaller. even though the garage portion of the save is larger in [size=+1]GT2[/size]. (I.e. both games have basic entries for 100 cars in the garage, and each basic entry is larger in [size=+1]GT2[/size]).
As far as I know, PS1 saves can't grow. E.g. when you choose a replay file (or settings file) size, if it's too small, you can't grow it later. (Although in [size=+1]GT2[/size] you could create a new larger replay file on another memory card, and copy all your replays to it).
If I recall correctly, in [size=+1]GT1[/size] there is a complete "parts inventory" in the save. For every part in the game, it records whether you own it or not. (One can argue that that is part of the garage, and therefore the garage is larger). (Suppose in [size=+1]GT1[/size], you obtain a certain model of car, purchase all its parts, and then sell the last instance of the car. If you later obtain another instance of that same model, all [removable] parts will be available for it). In [size=+1]GT2[/size], since you must purchase all parts for each car, that table is not necessary. (Instead each garage entry has room to indicate purchased parts; that's 100 versus 170 or so (and would be more like 500 in [size=+1]GT2[/size])).