Any process in which the symbiont activates or maintains its progression from an initial condition to a later condition, within the cells or tissues of the host organism. The host is defined as the larger of the organisms involved in the symbiotic interaction.
Comment:
This term partially replaces the obsolete term 'positive regulation of growth or development of symbiont in host ; GO:0033666'. See also 'positive regulation of growth of symbiont in host ; GO:0044128'.