Actually N64 games were usually much bigger than PS1 games.
Patently false. Total file sizes for PS1 games were typically far larger than the capacity of even the largest N64 cartridges. Super Mario 64 was on an 8MB cart, GoldenEye 007 was about 12MB, and Perfect Dark was on a 32MB cart (ROM sites peg the total file size at about 28.6MB). The biggest N64 carts could hold up to 64MB. Meanwhile, most PS1 games were well over 100MB, a problem for an N64 cart but no problem for CDs which could hold 700MB of data. Furthermore, CDs cost only a buck or two to manufacture while an N64 cart cost on the order of $15 or so.
All that extra space on CD-ROMS went to silly looking FMVs and voice acting lol. Cringeworthy voice acting might I add. The FMVs worked wonders to convince people to buy the PS1 though, that can't be denied. Also the much cheaper option, something that also can't be denied.
You can call FMVs "silly" or whatever you want. The point is is that many developers wanted them, because even 20 years ago many of them had a desire to add more "cinematic" elements in games. The level of detail and scope in FFVII's cinema scenes was something I thought was mind-blowing at the time, and the N64 most certainly couldn't pull off anything like them. Most developers were going to go with the best format there was that could fit their needs, and for the fifth generation that was CDs. The move away from cartridges was inevitable. They cost too much and couldn't hold enough data. Both today and back in the latter half of the 90s the N64 was to me by far the better fifth-gen system because I like its overall library more, but despite my preference for the N64 I can look back at it and see why third parties began flocking to PS1, and it was because Nintendo stuck with a format they wrongly thought still had some relevance in the console market. Sure, carts had no load times, were harder to pirate, and were more durable, but cost and capacity were the deciding factors that ensured discs were the future.