I shoot video in H.264. I assume that my footage is forever compressed really small and that exporting to ProRes in FCPX would do nothing but make my file bigger. Is this true? Or am I missing something? Would exporting in ProRes give me better quality than I started with?
Your source sets the quality ceiling, so transcoding to ProRes won't improve the video quality.
Most likely, the H264 files generated by your camera will be constrained baseline profile i.e. I- and P-frames only, and on a modern machine, decoding shouldn't be an issue. I edit H264 (baseline or main profile) all the time on my i7 and occasionally i5 with nVidia CUDA-enabled GPUs.
ProRes is useful when your shoot is in some exotic or heavier-duty codec because of bitrate or compression complexity. Or you need to collaborate with someone using FCP and they prefer to receive ProRes.
The 2 formats have different uses. H.264 is mainly good as a playback format. It's supported widely, compresses very well, and can be decompressed by relatively low-powered devices easily. It's also supported in some video cards making it hardware accelerated on those devices. While it is possible to edit with H.264, it's not ideal as it's a Long-GOP format. GOP here is an acronym for "Group of Pictures". Typically, Long-GOP formats contain 1 keyframe followed by several frames that are differences from the keyframes every so many seconds. So if you want to cut on a frame other than a keyframe, you need to decode the keyframe and all frames from the keyframe up to the frame you want to cut on, making it slower to edit.
ProRes, on the other hand, is intended to ensure the fidelity of the frame through numerous edits. While its compression is good, it focuses more on quality so the amount of compression is less than that of H.264. (In fact, in ProRes 4:4:4(:4) you can have lossless compression with ProRes.) Many devices will not playback ProRes because there's no codec written for them. I think that it's also a higher bit-depth than H.264. (There are higher bit-depth versions of H.264, but I don't know how widely supported they are.) It's also an intra-frame compression scheme, meaning that you don't need any previous frames to decode a given frame. That makes the compression less, but makes it easier to decode any given frame.
So ProRes is better for editing because it preserves fidelity, is faster to decode frames, and offers higher bit depths (or at least offers them more consistently). H.264 is better for playback because it's supported on more devices, and the compression ratio is higher.
You can think of it similarly to audio sample rates. For playback, nothing over 44.1 kHz can make any audible difference to humans (despite what marketing may tell you). But for the purposes of editing, working at 96 kHz can be useful for preserving fidelity through the addition of effects, various mix downs, and mastering.