Database growing fast with transients

Since v8.1 I get a lot of paired records in DB, e.g.

_transient_timeout_yst_sm_page_1:blpR_5ncLu
_transient_yst_sm_page_1:blpR_5ncLu

all stating something like:
C:24:”WPSEO_Sitemap_Cache_Data”:369:{a:2:{s:6:”status”;s:2:”ok”;s:3:”xml”;s:322:”<sitemapindex xmlns=”https://www.sitemaps.org/schemas/sitemap/0.9″&gt;