Throughout Genesis 1 and 2 there is a metaphor linking life on earth with our spirit.

When  earth/waters bring forth they symbolise what God the Father and Jesus/ The Holy Spirit bring forth respectively. There is emphasis on an ‘abundance’ of these gifts in day 5. The smaller creatures seem to refer to constituents and the larger attributes.

Could it be that to reflect  Adam's pre-fall status with huge gifts and power  over all the creation (Genesis 1:26-28) that God makes life on the first creation larger (dinosaurs, huge trees etc and ‘smaller ‘crawling life’ more abundant to parallel this (e.g. the Cambrian).

Much more on this in the Genesis Similitudes on this site here