>>546414016
This doujin manga was made around 2012-2013 so the size is more or less correct for the time
http://ashitahadocchida.sakura.ne.jp/saki.html
The creator released a Zenkoku-hen sequel in 2014 but nowadays they are just making Umamusume fanarts
https://www.pixiv.net/en/artworks/45457555