If I think back 20-25 years I remember really struggling with developing my pencil thumbnails into larger pieces. A thumbnail is sort of in flux allowing you to imagine it in various cool ways. Once you start refining, that aspect is lost as you're forced to make choices and fill in sensible details.
I tried scanning, printing out big, then doing new lines on top, but it somehow didn't work. Ultimately, I think I just didn't have enough experience thinking at a higher level of detail and also handle tricky stuff like anatomy and gesture.
But, I did manage to gain some leverage when I discovered that I could paint right over an upscaled thumbnail digitally. Often I'd multiply-brush some muddy base colour over the entire canvas or figure, then paint in details using mostly additive (lighter) transparent colours. This worked really well up until a point. I could make a piece look finished, and I didn't necessarily lose the "flux" quality of the thumbnail as I could leave areas vague in a painterly fashion.
However, it was still difficult and time consuming for me to paint and change details this was because of the number of brushstrokes that were needed. It felt like I was missing some piece... like there was friction in my process I should try to get rid of. Also, I often failed to address problems inherent in the thumbnails as I would not really alter it much, just paint over. At other times I got so caught up in doing brush strokes, painting over and nudging that I failed to notice that I had drifted off, sabotaging some important flow, silhouette or detail present in the original thumbnail.
Then around 2006-2010 I gradually abandoned that workflow and started doing more line art and spending time on the figure (rather than being locked up by adjusting paint blobs). I had recently worked as a creative consultant on a big project that would later become Wreck-it-Ralph, forcing me to consider new ways of drawing and being less haphazard and more efficient with lines. It's when I did a lot of the chibi stuff on my site. After exploring that path for a few years and learning a few things, I decided/realized that cleaner lines were actually missing that "flux" that I liked, so that's why my recent pieces are have more of a pencil look, all fuzzy and scratchy.
Then there's my lack of patience. When a piece just about works... I drop it so I can so another one. So that's where I'm at now... I'm also not afraid of going back to old pieces with new insights. Everything is a WIP. In 2016 I went back to a 2006 Chozo piece, just fiddling a bit with it, though it's very much still in my early 200X style.
The thumbnails here were scanned on my (1999?) AGFA SnapScan 1236u, which died only a few years ago, so I bought a cheap Canon LiDE 300. It's dwarfed by the AGFA unit, but quality is okay. It picks up the paper texture, which has always been my litmus test. I've had a couple of scanners fail it miserably, so I returned them and stuck with the AGFA for ages. But it has moved on to being a cat bed/platform/pedestal now.