Thanks all!
@DNSP: I'm aware that Dropcaps are no good idea for epubs. For a time, I was experimenting myself with them, but they render different on each device, and even on one single device it depends on the individual settings. But larger first letters are a different thing. A similar problem with too large a line space occurs with <sup>-tags. I usually achieved satisfactory results by setting the line-height to 0, but in the case described there is no difference.
@Karellen: I'll give that a try, thank you. But what makes me wonder is that the code I use makes no problem on other devices, only on the Clara. I suspect that there is a sort of Kobo pre-setting that causes the issue rather than an erroneous code.
@JSWolf: I don't know how larger first letters look on Kindle, but I would be interested, too, to have a look at it. Would you kindly (Kindle-y
) try with my code or look hat one of my books (they are epubs, however)?